Luiggi Caldas • August 7, 2026

Subordinate Clause: Types, Comma Rules, and Common Errors

Read this out loud: Because I forgot my keys.

Something feels unfinished. The words have a subject ( I ) and a verb ( forgot ), and they make sense — but they leave you waiting for the rest. Because you forgot your keys, what happened?

That is a subordinate clause. It has everything a sentence needs except the ability to stand alone.

Subordinate clauses are how English combines ideas. Without them, you write in short, disconnected statements. With them, you can show cause, condition, time, contrast, and relationship — all inside a single sentence. They are the structural difference between beginner writing and fluent writing.

They also produce two of the most common errors in English: the sentence fragment, and the wrong verb tense after words like when and if.

This guide covers both.

Contents

  1. What is a subordinate clause?
  2. Clause vs. phrase
  3. Independent vs. subordinate
  4. How to recognize one: the trigger words
  5. The three types
  6. The comma rule
  7. Sentence fragments: the error to avoid
  8. Verb tense in subordinate clauses
  9. When you can omit "that"
  10. Common mistakes for English learners
  11. Practice exercises

1. What Is a Subordinate Clause?

A subordinate clause — also called a dependent clause — is a group of words that contains a subject and a verb but cannot stand alone as a complete sentence. It must be attached to an independent clause to make sense.

Examples:

  • I stayed home ***because it was raining****.*
  • ***When the movie ended****, we went to dinner.*
  • The book that you recommended was excellent.
  • She told me ***what she wanted****.*

In each sentence, the bolded portion has a subject and a verb, but on its own it would leave the reader waiting.

The word subordinate is literal here: the clause is placed under another clause in importance. It supports the main idea rather than carrying it.

2. Clause vs. Phrase

This distinction is foundational, and confusing them is a frequent source of error.

A clause has a subject and a verb. A phrase does not.

Example Subject Verb
Phrase on the table — —
Phrase running down the street — —
Phrase a retired pilot — —
Clause because she was tired she was
Clause when the bus arrives the bus arrives
Clause that I bought yesterday I bought

This matters because phrases and clauses follow different punctuation rules and do different jobs in a sentence.

For phrases that modify nouns, see our guides on appositive phrases and participial phrases.

3. Independent vs. Subordinate

Every clause is one of two kinds.

An independent clause expresses a complete thought and can stand alone as a sentence.

  • She works at the hospital.
  • The train left.
  • I understand.

A subordinate clause contains a subject and a verb but does not express a complete thought.

  • Although she works at the hospital…
  • After the train left…
  • …that I understand.

Here is the useful insight: a subordinate clause is often just an independent clause with a word added to the front.

Independent Add a word Subordinate
She was late. because because she was late
The rain stopped. when when the rain stopped
He agreed. although although he agreed

That single added word — the subordinating conjunction — is what removes the clause's independence. Remove the word and the clause stands again.

4. How to Recognize One: The Trigger Words

Subordinate clauses almost always begin with one of two kinds of words.

Subordinating conjunctions

Grouped by the relationship they express:

Relationship Conjunctions
Time when, whenever, while, before, after, since, until, as soon as, once, as
Cause / reason because, since, as, now that, in that
Condition if, unless, provided that, in case, as long as, even if
Contrast although, though, even though, whereas, while
Purpose so that, in order that
Result so… that, such… that
Manner as, as if, as though
Place where, wherever

Relative pronouns and relative adverbs

Relative pronouns: who, whom, whose, which, that, whoever, whomever, whatever

Relative adverbs: when, where, why

  • The woman who called is my aunt.
  • The house where I grew up was demolished.
  • The reason why he left is unclear.

The practical test: if a group of words has a subject and a verb but starts with one of these trigger words, it is a subordinate clause and cannot stand alone.

5. The Three Types

Subordinate clauses are classified by what job they do in the sentence.

Adverb clauses

These modify a verb, adjective, or another adverb. They answer when, where, why, how, under what condition. They begin with subordinating conjunctions.

  • I'll call you ***when I arrive****.* (when)
  • She left ***because she was tired****.* (why)
  • ***If you finish early****, let me know.* (condition)
  • He speaks ***as though he knows everything****.* (how)

Adverb clauses are the most mobile — they can go at the beginning or the end of a sentence without changing the meaning.

Adjective clauses (relative clauses)

These modify a noun or pronoun. They usually begin with a relative pronoun.

  • The car that I bought was expensive.
  • My sister, ***who lives in Boston****, is a nurse.*
  • The restaurant where we met has closed.

Adjective clauses sit directly after the noun they describe. They cannot be moved.

Noun clauses

These function as a noun — as the subject, object, or complement of the sentence.

  • What she said surprised everyone. (subject)
  • I don't know ***where he went****.* (object)
  • The problem is ***that we're out of time****.* (complement)

How to identify one: replace the whole clause with it or something. If the sentence still works, it's a noun clause.

What she said surprised everyone. → It surprised everyone. ✓ Noun clause.

6. The Comma Rule

This rule is clean, testable, and covers the vast majority of cases.

Subordinate clause first → use a comma

  • ***Because it was raining****, we canceled the picnic.*
  • ***When she arrives****, tell her to wait.*
  • ***Although he studied hard****, he failed the exam.*

Main clause first → no comma

  • We canceled the picnic ***because it was raining****.*
  • Tell her to wait ***when she arrives****.*
  • He failed the exam ***although he studied hard****.*

Same sentence, same meaning, different punctuation depending on order.

The exception: contrast clauses

Clauses beginning with although, though, even though, whereas , and while (meaning "whereas") often take a comma even when they come second, because they express contrast rather than continuation.

  • She kept working, ***although she was exhausted****.*
  • He prefers coffee, ***whereas I prefer tea****.*

Adjective clauses follow a different rule

For relative clauses, the comma depends on whether the information is essential — the same restrictive/nonrestrictive distinction that governs appositives.

Essential (no commas): The students who submitted late received a penalty. The clause identifies which students. Remove it and the meaning changes.

Nonessential (commas): My brother, ***who lives in Chicago****, is visiting next week.* If you have one brother, the clause is extra information.

A useful signal: that introduces essential clauses. Which usually introduces nonessential ones in American English.

  • The book that I ordered arrived. (essential, no commas)
  • The book, which I ordered last week, arrived. (nonessential, commas)

7. Sentence Fragments: The Error to Avoid

A sentence fragment happens when a subordinate clause is punctuated as if it were a complete sentence.

❌ I couldn't go to the party. Because I had to work. ✅ I couldn't go to the party ***because I had to work****.*

❌ Although she tried her best. She didn't win. ✅ ***Although she tried her best****, she didn't win.*

❌ He bought a new car. Which was very expensive. ✅ He bought a new car, ***which was very expensive****.*

Why this error is so common

Subordinate clauses feel complete. Because I had to work has a subject, a verb, and a clear meaning. The instinct to end it with a period is natural.

The test: read the clause alone, out loud. If it leaves you waiting for more information, it is a fragment. Attach it to the sentence before or after it.

The deliberate exception

Skilled writers occasionally use fragments for emphasis in creative or informal writing:

She read the letter twice. Then a third time. Because she couldn't believe it.

This works in fiction and journalism. In academic writing, professional communication, and standardized exams, avoid it — the reader cannot tell whether the fragment was intentional or a mistake.

8. Verb Tense in Subordinate Clauses

This section matters more than any other for speakers of Spanish, Portuguese, French, and Italian, because the rules differ significantly from those languages.

Rule 1 — No future tense after time conjunctions

After when, before, after, until, as soon as, once , and while , English uses present tense to talk about the future.

❌ When I ***will arrive****, I will call you.* ✅ When I ***arrive****, I will call you.*

❌ I'll wait until she ***will finish****.* ✅ I'll wait until she ***finishes****.*

❌ As soon as the meeting ***will end****, we'll leave.* ✅ As soon as the meeting ***ends****, we'll leave.*

The future is expressed once, in the main clause. The subordinate clause stays in the present.

Why this is hard for Spanish and Portuguese speakers: those languages use the subjunctive here ( Cuando llegue , Quando eu chegar ), which is a distinct verb form. English has no equivalent form, so learners either use the future by analogy or hesitate.

The fix is simple: after a time conjunction, use the present simple.

Rule 2 — The same applies to conditional if

❌ If it ***will rain****, we'll stay home.* ✅ If it ***rains****, we'll stay home.*

Note the difference between real and unreal conditions:

  • Real condition: If it rains, we'll stay home. (present + will)
  • Unreal condition: If it rained, we would stay home. (past + would)
  • Impossible past: If it had rained, we would have stayed home. (past perfect + would have)

Rule 3 — Backshifting in reported speech

When the main clause is in the past, the subordinate clause usually shifts back one tense.

Direct Reported
"I am tired." He said he was tired.
"I work here." She said she worked there.
"I will call." He said he would call.
"I have finished." She said she had finished.

Exception: if the statement is still true or is a general fact, backshifting is optional.

He said that water boils at 100 degrees. (still true — no shift needed)

Rule 4 — English has a subjunctive, but a small one

Spanish uses the subjunctive constantly in subordinate clauses. English uses it in only two situations.

The mandative subjunctive — after verbs of suggestion, demand, or requirement, the verb stays in base form with no -s :

  • I suggest that he be informed. (not is )
  • The manager insisted that she arrive early. (not arrives )
  • It's essential that everyone submit the form. (not submits )

The***were*****-subjunctive** — in hypothetical situations:

  • If I were you, I'd take the job. (not was )
  • She acts as if she were the boss.

Outside these two cases, English uses the indicative where Spanish would use the subjunctive:

Spanish: Espero que ***venga****.* (subjunctive) English: I hope he ***comes****.* (indicative — normal present)

9. When You Can Omit "That"

In several situations that is optional. Knowing when makes your English sound more natural.

Optional after reporting verbs

  • I think (that) she's right.
  • He said (that) he was busy.
  • We know (that) it works.

In conversation, that is usually dropped. In formal writing, it is usually kept.

Optional when the relative pronoun is the object

  • The book (that) I read was excellent. — I is the subject, that is the object. Optional.
  • The person (whom) you met is my boss. Optional.

Required when the relative pronoun is the subject

  • The book that won the prize was excellent. — that is the subject of won. Required.
  • The man who called was my neighbor. Required.

The test: if another subject follows immediately, that can go. If a verb follows immediately, that must stay.

The book that I read… → subject I follows → optional The book ***that won****…* → verb won follows → required

Never omit after certain constructions

  • After so… that : It was so cold that we stayed inside.
  • When the clause is the subject: That he lied surprised no one.
  • After a comma in a nonrestrictive clause (use which , not that )

10. Common Mistakes for English Learners

Mistake 1 — Future tense after time conjunctions

❌ When I will finish, I will call you. ✅ When I finish, I will call you.

The most persistent error for Romance language speakers. Covered in section 8.

Mistake 2 — Sentence fragments

❌ I didn't come. Because I was sick. ✅ I didn't come because I was sick.

Mistake 3 — Double conjunction

Spanish and Portuguese allow structures that English does not.

❌ Although he was tired, but he kept working. ✅ Although he was tired, he kept working. ✅ He was tired, but he kept working.

Use one connector, not two. The same applies to because… so :

❌ Because it was late, so we left. ✅ Because it was late, we left.

Mistake 4 — Wrong comma placement

❌ We canceled the trip, because it was raining. ✅ We canceled the trip because it was raining.

No comma when the main clause comes first.

Mistake 5 — Question word order inside a noun clause

❌ I don't know ***where is he****.* ✅ I don't know ***where he is****.*

A noun clause keeps statement word order, even when it begins with a question word. See our guide on interrogative sentences for the full explanation.

Mistake 6 — Omitting the subject

Spanish and Portuguese allow the subject to be dropped. English does not.

❌ When arrived, she called me. ✅ When she arrived, she called me.

Mistake 7 — Using the subjunctive where English uses the indicative

❌ I hope that he come tomorrow. ✅ I hope that he comes tomorrow.

11. Practice Exercises

Exercise 1 — Identify the subordinate clause

Find the subordinate clause in each sentence.

  1. We stayed inside because the weather was terrible.
  2. The woman who lives next door is a doctor.
  3. When the alarm rang, everyone left the building.
  4. I don't know what she wants.
  5. Although the restaurant was expensive, the food was disappointing.
  1. because the weather was terrible (adverb clause)
  2. who lives next door (adjective clause)
  3. When the alarm rang (adverb clause)
  4. what she wants (noun clause)
  5. Although the restaurant was expensive (adverb clause)

Exercise 2 — Fix the fragment

Each item contains a fragment. Rewrite it as one correct sentence.

  1. She didn't answer the phone. Because she was in a meeting.
  2. Although we arrived early. There were no seats left.
  3. He bought a used car. Which broke down after a week.
  4. I'll let you know. As soon as I hear back.
  5. When the results came in. Everyone celebrated.
  1. She didn't answer the phone because she was in a meeting.
  2. Although we arrived early, there were no seats left.
  3. He bought a used car, which broke down after a week.
  4. I'll let you know as soon as I hear back.
  5. When the results came in, everyone celebrated.

Exercise 3 — Add commas where needed

Some sentences need a comma. Some don't.

  1. When the meeting ends we'll go to lunch.
  2. We'll go to lunch when the meeting ends.
  3. Because the flight was delayed I missed my connection.
  4. I missed my connection because the flight was delayed.
  5. My laptop which I bought last year already needs repairs.
  1. When the meeting ends *,** we'll go to lunch.*
  2. We'll go to lunch when the meeting ends. (no comma)
  3. Because the flight was delayed *,** I missed my connection.*
  4. I missed my connection because the flight was delayed. (no comma)
  5. My laptop *,** which I bought last year**,** already needs repairs.*

Exercise 4 — Choose the correct verb form

  1. I'll call you when I (arrive / will arrive).
  2. If it (rains / will rain) tomorrow, we'll cancel.
  3. She said she (is / was) tired.
  4. I suggest that he (be / is) informed immediately.
  5. If I (was / were) you, I'd accept the offer.
  6. We'll wait until she (finishes / will finish).
  1. arrive — no future after time conjunctions
  2. rains — no future after conditional if
  3. was — backshifting in reported speech
  4. be — mandative subjunctive after suggest
  5. were — subjunctive in hypothetical situations
  6. finishes — no future after until

Exercise 5 — Combine the sentences

Use a subordinating conjunction to join each pair.

  1. It started raining. We went inside.
  2. She studied for months. She didn't pass.
  3. The store closes at nine. Get there before then.
  4. He was nervous. He gave an excellent presentation.
  5. I finish work at six. I'll meet you afterward.
  1. When it started raining, we went inside.
  2. Although she studied for months, she didn't pass.
  3. Get there before the store closes at nine.
  4. Even though he was nervous, he gave an excellent presentation.
  5. After I finish work at six, I'll meet you.

Frequently Asked Questions

What is a subordinate clause in simple terms?

A group of words with a subject and a verb that cannot stand alone as a sentence. Because it was raining has a subject and a verb but leaves you waiting for more — it needs a main clause to complete the thought.

What is the difference between a subordinate clause and a dependent clause?

There is no difference. They are two names for the same thing. Subordinate clause is more common in British grammar terminology; dependent clause in American.

What is the difference between a clause and a phrase?

A clause contains a subject and a verb. A phrase does not. Because she was tired is a clause. On the table and running down the street are phrases.

Do subordinate clauses always need a comma?

No. Use a comma when the subordinate clause comes before the main clause. Use no comma when it comes after. Adjective clauses follow a different rule based on whether the information is essential.

Why can't I say "When I will arrive"?

Because English does not use the future tense after time conjunctions like when, before, after, until, and as soon as. The future is expressed once, in the main clause: When I arrive, I will call you.

What is a sentence fragment?

A subordinate clause punctuated as if it were a full sentence. Because I was late. is a fragment. Attach it to the sentence it belongs with: I missed the bus because I was late.

When can I leave out "that"?

When it introduces a reported statement ( I think she's right ) or when the relative pronoun is the object of the clause ( The book I read ). Keep that when it is the subject of the clause ( The book that won the prize ).

References

  1. Cambridge Dictionary. Clauses and sentences. https://dictionary.cambridge.org/grammar/british-grammar/clauses-and-sentences
  2. Purdue OWL. Independent and Dependent Clauses. https://owl.purdue.edu/owl/general_writing/mechanics/independent_and_dependent_clauses/index.html
  3. Swan, Michael. Practical English Usage. Oxford University Press.
  4. Murphy, Raymond. English Grammar in Use. Cambridge University Press.
  5. British Council. LearnEnglish — Grammar reference. https://learnenglish.britishcouncil.org/grammar

Published by Lingua Language Center — ACCET-accredited and SEVP-certified English language school in South Florida, teaching English and foreign languages since 1998. For more free English resources, visit lingua.edu.