A complex sentence has one main clause that can stand alone and at least one dependent clause that cannot. The dependent clause starts with a word such as although, because, when, if, who or which. Complex sentences let you show reasons, contrasts and conditions in one sentence.
Simple, compound and complex
A clause is a group of words with a subject and a verb. A simple sentence has one clause: Cities are growing. A compound sentence joins two equal clauses with a word such as and, but or so: Cities are growing, and they need more homes.
A complex sentence joins clauses that are not equal. One is the main clause. The other depends on it and cannot stand alone: Although cities are growing, many villages are becoming smaller. The part starting with Although is the dependent clause. It does not make sense by itself.
Good writing mixes all three. Short sentences give clear facts. Compound sentences add similar ideas. Complex sentences show how ideas are connected.
- Cities are growing. · Cities are growing, and they need more homes.
- Cities are growing, and they need more homes. · Although cities are growing, many villages are becoming smaller.
The words that start a dependent clause
Dependent clauses start with a joining word. These words fall into three groups, and learning them by job makes them easier to remember.
Adverbial clauses answer questions such as when, why or in what case. give more detail about a noun. Noun clauses act as the subject or object of the main verb.
- Contrast: although, even though, while, whereas
- Reason: because, since, as
- Time: when, after, before, while, until
- Condition: if, unless, as long as
- Relative: who, which, that, whose, where
- Noun clauses: that, whether, what, how
Punctuation and word order
When the dependent clause comes first, put a comma after it: Because the road was closed, we took the train. When the main clause comes first, a comma is usually not needed: We took the train because the road was closed.
Never add a second joining word. Although it was expensive, but we bought it is wrong. Choose one: Although it was expensive, we bought it, or It was expensive, but we bought it. The same applies to because and so: say Because he was tired, he went home, not Because he was tired, so he went home.
In , commas depend on the meaning. If the clause gives extra information, use commas: My brother, who lives in Madrid, is an engineer. If it identifies which person or thing, use no commas: The engineer who designed the bridge won an award.
Avoiding fragments and run-ons
A fragment is a dependent clause written as a full sentence. Although the course was expensive. is a fragment because it needs a main clause. Join it to the previous or next sentence: Although the course was expensive, it was worth the cost.
A run-on joins two main clauses with only a comma or with nothing. The city is crowded, it is also expensive is a comma splice. Fix it with a full stop, with a joining word such as and or but, or with a semicolon.
Check each long sentence by asking: Does it have a main clause that could stand alone? Do the clauses join properly? These two questions find most problems.
Build one step at a time
If complex sentences feel hard, build them in steps. Start with a simple sentence: Online learning is popular. Ask why, and add a reason: Online learning is popular because it is flexible. Ask about a limit, and add a contrast: Online learning is popular because it is flexible, although some students feel isolated.
Stop when the sentence is clear. Two dependent clauses in one sentence are usually enough. If you add three or four, the main idea is lost, and the reader has to work too hard. Remember that your aim is to show the link between ideas, not to make the sentence long.
Using complex sentences in IELTS
In Writing Task 2, complex sentences show that you can link reasons, concessions and examples. In Task 1, use them to compare data: While car use has fallen in the city, it has risen in the countryside.
In Speaking, you do not need long sentences all the time. A natural complex sentence is easier to say than it looks if you plan the first clause and let the second one follow: I enjoy living in a big city because there is always something to do.
Who finds this hard?
- Chinese speakers: Chinese often places the dependent clause first and links ideas by meaning alone, so learners may leave out the joining word or add a second one.
- Japanese speakers: Japanese puts the main verb last and before the noun, so English relative clauses after the noun need practice.
- Korean speakers: Korean also places modifying clauses before the noun and ends clauses with verb endings, which affects relative clauses in English.
- Arabic speakers: Arabic often joins clauses with and, so learners write long chains of and instead of using a dependent clause.
IELTS tip
Do not write every sentence as a complex sentence. Mix short and complex sentences, and use a complex sentence when you want to give a reason, a contrast or a condition. Check each one for a main clause and for a comma after an opening dependent clause.
Frequently asked questions
What is a complex sentence?
It is a sentence with one main clause and at least one dependent clause. The dependent clause starts with a word like although, because, if or who.
Do I always need a comma?
Use a comma after a dependent clause that comes first. Usually no comma is needed when it comes second.
Is a longer sentence always better?
No. Clear sentences matter more than long ones. Use complex sentences when you need to show a link between ideas.
Can I start a sentence with because?
Yes, if you finish the sentence with a main clause. Because the road was closed, we took the train is complete.
Sources
- Carter, R. and McCarthy, M. (2006). Cambridge Grammar of English. Cambridge University Press.
- Biber, D., Johansson, S., Leech, G., Conrad, S. and Finegan, E. (1999). Longman Grammar of Spoken and Written English. Longman.