Complex sentences
A complex sentence is a sentence that contains one independent clause (a complete thought that can stand alone) and at least one dependent clause (an incomplete thought that relies on the independent clause). Dependent clauses often begin with words like because, although, if, when or who, and they add extra detail, reason, time, condition or contrast to the main idea.
Types of complex sentences
Adverbial clause complex sentences
Contain a dependent clause that acts like an adverb, showing time, reason, condition, contrast or place.
e.g. Because it was raining, we stayed inside., She left early although she wanted to stay longer.
Relative (adjective) clause complex sentences
Contain a dependent clause introduced by a relative pronoun that describes a noun in the main clause.
e.g. The teacher who marked my essay gave useful feedback., This is the city where I was born.
Noun clause complex sentences
Contain a dependent clause that functions as a subject or object, often starting with that, what, whether or a question word.
e.g. What she said surprised everyone., I don't know whether he will come.
Conditional complex sentences
Use an if-clause (dependent) with a main clause to express real, unreal or hypothetical situations.
e.g. If the weather improves, we will go hiking., If I had studied harder, I would have passed.
Rules to remember
- A complex sentence needs exactly one independent clause plus one or more dependent clauses; it cannot consist of only dependent clauses.
- When the dependent clause comes first, use a comma before the independent clause; when it comes second, a comma is usually not needed.
- Subordinating conjunctions (because, although, since, if, when, while) or relative pronouns (who, which, that) are used to introduce the dependent clause and show the relationship between ideas.
- The dependent clause must contain a subject and a verb but still cannot stand alone as a complete sentence.
- Do not confuse complex sentences with compound sentences: compound sentences join two independent clauses with a coordinating conjunction (and, but, so), while complex sentences join an independent and a dependent clause.
Examples in sentences
| Example | How it works |
|---|---|
| Although the exam was difficult, most students passed. | The dependent clause 'Although the exam was difficult' precedes the independent clause, so a comma is used. |
| I will call you when I arrive at the airport. | The independent clause comes first, so no comma is needed before the dependent time clause. |
| The book that you recommended was excellent. | A relative clause introduced by 'that' describes the noun 'book'. |
| She didn't attend the meeting because she was ill. | The dependent clause gives the reason for the action in the independent clause. |
| Whoever finishes first will receive a prize. | A noun clause acts as the subject of the independent clause. |
| If prices continue to rise, many families will struggle financially. | A conditional (if) clause functions as the dependent clause showing a likely future result. |
| The manager, who has worked here for ten years, is retiring next month. | A non-defining relative clause adds extra information about the subject, set off by commas. |
Common mistakes
Incorrect: Because I was tired.
Correct: Because I was tired, I went to bed early. (A dependent clause alone is a sentence fragment; it must be joined to an independent clause.)
Incorrect: I went to bed early, because I was tired I couldn't focus.
Correct: I went to bed early because I was tired; I couldn't focus. (Do not join two independent clauses with only a comma; use a semicolon, full stop or conjunction.)
Incorrect: The report, that I wrote it, was accepted.
Correct: The report that I wrote was accepted. (Do not repeat the subject with a pronoun after a relative clause.)
Why this matters for IELTS
Using complex sentences accurately is one of the clearest ways to raise your Grammatical Range and Accuracy score in IELTS Writing and Speaking. Examiners look for a mix of sentence types, and complex sentences show you can link ideas logically using reason, contrast, condition and description. However, accuracy matters more than complexity: a few well-formed complex sentences with correct punctuation and clause structure will score higher than many long, confusing ones with fragment or run-on errors.
Frequently asked questions
What is a complex sentence in simple terms?
It is a sentence made of one main idea that can stand alone (independent clause) joined with one or more extra ideas that cannot stand alone (dependent clauses), connected by words like because, although, if or who.
What is the difference between a complex sentence and a compound sentence?
A compound sentence joins two independent clauses with a coordinating conjunction like and, but or so, while a complex sentence joins an independent clause with at least one dependent clause using words like because, although or which.
Can a complex sentence have more than one dependent clause?
Yes, a complex sentence can contain two or more dependent clauses alongside its single independent clause, for example: Although it was raining, we went outside because we needed exercise.
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