A complex sentence combines one main idea with an extra piece of information that depends on it, as in She stayed home because she was ill. Writers use this structure all the time to show reasons, conditions, contrasts and time relationships, yet learners often mix it up with the compound sentence or accidentally write a fragment instead of a full sentence. Understanding how the two clauses fit together makes it much easier to write clearly and to punctuate correctly.
This article explains what a complex sentence is, the structure it follows, the connecting words that build it, how it differs from a compound sentence, what a compound-complex sentence is, and gives exercises with answers.
What is a complex sentence? Definition and the two kinds of clause it contains
The basic definition of a complex sentence and what it contains
A complex sentence contains one independent clause and at least one dependent clause. An independent clause has a subject and a verb and can stand alone as a complete sentence. A dependent clause also has a subject and a verb, but it cannot stand alone because it begins with a word that makes it rely on the main clause.
Example: Although it was raining, we went for a walk. – “Although it was raining” is the dependent clause, and “we went for a walk” is the independent clause.
Why the dependent clause cannot stand alone
A dependent clause leaves the listener or reader waiting for the main idea. If you say only Although it was raining, the sentence feels unfinished, which is why a dependent clause written alone is treated as a sentence fragment.
Complex sentence structure: The two possible word orders and their punctuation
The complex sentence structure can be arranged in two ways, and the order affects the punctuation.
| Order | Pattern | Example |
| Dependent clause first | Dependent clause + comma + independent clause | Because it was late, we went home. |
| Independent clause first | Independent clause + dependent clause, usually no comma | We went home because it was late. |
The comma rule that depends on which clause comes first
When the dependent clause comes first, a comma separates it from the main clause. When the main clause comes first, a comma is normally not needed, although writers sometimes add one before words of strong contrast such as although or whereas.
Examples: If you study hard, you will pass the exam. You will pass the exam if you study hard.
Subordinating conjunctions that build complex sentences, grouped by meaning
A dependent clause is usually introduced by a subordinating conjunction. The table groups common ones by the relationship they show.
| Relationship | Subordinating conjunctions | Example |
| Time | when, while, before, after, since, until, as soon as | I will call you when I arrive. |
| Reason | because, since, as | He left early because he felt ill. |
| Contrast | although, though, even though, whereas, while | Although it was cold, they went swimming. |
| Condition | if, unless, provided that | Unless you hurry, you will miss the bus. |
| Purpose | so that, in order that | She spoke slowly so that everyone could understand. |
| Place | where, wherever | We stayed where the view was beautiful. |
Three types of dependent clause found in complex sentences
Adverbial clauses that show time, reason, contrast or condition
These give information about time, reason, contrast, condition or purpose, and they are introduced by the subordinating conjunctions above.
Example: She smiled because she had good news.
Relative (adjective) clauses that describe a noun in the main clause
These describe a noun and are introduced by relative pronouns such as who, which, that, whose or relative adverbs such as where. You can read more about these connecting words in our article on the relative pronoun.
Example: The man who lives next door is a doctor.
Noun clauses that act like a noun inside the sentence
These work like a noun inside the sentence and often begin with that, whether, what, how or why.
Examples: I know that she is late. What he said surprised everyone.
Complex sentence examples that show different meanings in context
Here are several complex sentence examples, each showing a different kind of relationship:
- When the bell rang, the students left the classroom. (time)
- She passed the exam because she studied every day. (reason)
- Although he was tired, he finished the project. (contrast)
- If it rains tomorrow, we will cancel the picnic. (condition)
- The book that I borrowed was very interesting. (relative clause)
- I wonder whether he will come. (noun clause)
Complex sentence versus compound sentence: What separates the two structures
The difference lies in how the clauses are connected and how equal they are.
| Feature | Complex sentence | Compound sentence |
| Clauses | One independent + at least one dependent | Two or more independent |
| Connectors | Subordinating conjunctions, relative pronouns | Coordinating conjunctions (for, and, nor, but, or, yet, so), semicolons |
| Relationship | One idea depends on the other | Ideas are of equal weight |
| Example | I stayed home because I was ill. | I was ill, so I stayed home. |
Both sentences in the last row express a similar meaning, but the first makes the reason depend on the main idea, while the second presents two independent statements joined by so. For a full explanation of the second pattern, see our article on the compound sentence.
Compound complex sentence: What it is and how to build one
A compound complex sentence, also written as a compound-complex sentence, contains at least two independent clauses and at least one dependent clause.
Example: Although it was raining, we went for a walk, and we enjoyed it. – one dependent clause (“Although it was raining”) and two independent clauses (“we went for a walk” and “we enjoyed it”).
Another example: She stayed home because she was ill, but her brother went to school. Here “because she was ill” is the dependent clause, while “She stayed home” and “her brother went to school” are independent clauses joined by but.
A compound-complex sentence lets a writer combine several related ideas, but a long sentence of this type can become hard to follow, so each clause should be clear and the punctuation careful.
Common mistakes with complex sentences and how to fix them
- Writing a dependent clause as a full sentence. Because it was raining. is a fragment. Join it to a main clause: Because it was raining, we stayed inside.
- Using two connectors that do the same job. Writing Although he was tired, but he kept working instead of Although he was tired, he kept working. English does not use although and but together in the same sentence.
- Forgetting the comma after an introductory dependent clause. Writing When the bell rang the students left instead of When the bell rang, the students left.
- Repeating the subject or object in a relative clause. Writing The book that I bought it was expensive instead of The book that I bought was expensive.
- Choosing the wrong connector. Using because where although is needed changes the meaning completely, so check whether the clause gives a reason or a contrast.
Complex sentence exercises with answers to check your understanding
Exercise 1: Combine the two sentences into one complex sentence
- She was tired. She finished the report. (use although)
- It was raining. We stayed inside. (use because)
- You study hard. You will pass the exam. (use if)
- The man lives next door. He is a doctor. (use who)
Sample answers:
- Although she was tired, she finished the report.
- We stayed inside because it was raining.
- If you study hard, you will pass the exam.
- The man who lives next door is a doctor.
Exercise 2: Identify the type of each sentence
- I like tea. → Simple sentence
- I like tea, and she likes coffee. → Compound sentence
- I like tea because it relaxes me. → Complex sentence
- Although I like tea, I drink coffee in the morning, and she drinks juice. → Compound-complex sentence
Exercise 3: Find and correct the error in each sentence
- Because it was late. → correct to Because it was late, we went home.
- Although he was tired, but he kept working. → correct to Although he was tired, he kept working.
- When the bell rang the students left. → correct to When the bell rang, the students left.
- The book that I bought it was expensive. → correct to The book that I bought was expensive.
A complex sentence joins an independent clause to one or more dependent clauses, using subordinating conjunctions or relative pronouns to show how the ideas relate. Learning the two possible word orders, the comma rule and the common connectors helps you avoid fragments and write sentences that are varied but still easy to follow. Once you are comfortable with complex sentences, combining them with compound structures gives you the compound-complex pattern for longer, more detailed writing.
See also: Compound sentence: What it is and how to write one
Frequently asked questions
What is a complex sentence in simple terms?
A complex sentence is a sentence with one main clause that can stand alone and at least one dependent clause that cannot. The dependent clause usually starts with a word such as because, although, if, when or who.
Do I always need a comma in a complex sentence?
Not always. If the dependent clause comes first, use a comma after it, as in “Because it was late, we went home.” If the main clause comes first, a comma is normally not needed, as in “We went home because it was late.”
What is the difference between a complex sentence and a compound-complex sentence?
A complex sentence has one independent clause and at least one dependent clause. A compound-complex sentence has at least two independent clauses and at least one dependent clause, such as “Although it was raining, we went for a walk, and we enjoyed it.”
Keyword:
- compound complex sentence
- complex sentence exercises
- complex sentence structure
- compound-complex sentence
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