Complex Sentences for IELTS: How to Build Them Without Breaking Them

A complex sentence needs exactly one independent clause — a clause that could stand alone as a full sentence — plus at least one dependent clause, which cannot stand alone and relies on the independent clause for meaning. 'Because the flight was delayed' is a dependent clause; add 'we missed our connection' and the pair becomes a complete complex sentence.

This is different from a compound sentence, which joins two independent clauses with a coordinating conjunction (and, but, so) rather than subordinating one to the other. Examiners credit complex sentences more highly than compound ones, because subordination shows you can rank ideas by importance rather than simply listing them side by side.

Almost every complex sentence in IELTS Speaking and Writing is built from one of three tools: a subordinating conjunction, a relative clause, or a noun clause. Knowing which tool you're reaching for makes the punctuation and word order far easier to get right, and it also helps you notice when you keep defaulting to only one of the three — a sure sign your range needs broadening. Most candidates over-rely on subordinating conjunctions and under-use noun clauses, simply because noun clauses feel less familiar.

A comma splice happens when a writer senses that two ideas are connected and reaches for a comma to join them, without adding the conjunction or subordinator that actually links them grammatically. 'Costs at the factory rose sharply, output remained unchanged' looks connected in meaning but is two independent clauses glued together — grammatically, it needs a semicolon, a conjunction, or a subordinator, not a bare comma.

The fix is always one of three: add a coordinating conjunction after the comma (and, but, so), replace the comma with a semicolon if no conjunction is added, or turn one clause into a dependent clause with a subordinator. All three are equally correct — choose based on which relationship (addition, contrast, cause) you actually mean.

• Costs at the factory rose sharply, output remained unchanged.(comma splice — two independent clauses)
• Costs at the factory rose sharply, yet output remained unchanged.
• Although costs at the factory rose sharply, output remained unchanged.(subordinator turns one clause dependent)

A fragment is the opposite problem: a dependent clause punctuated as if it were a complete sentence, with no independent clause attached anywhere nearby. 'Which is why so many families are moving to the suburbs.' has no main clause of its own — it depends on a sentence before it that a reader can only guess at.

Fragments often appear when a candidate tries to add emphasis by starting a new sentence with 'Which', 'Because', or 'Although' and then forgets to attach the independent clause. The rule is simple: read the fragment alone and ask whether it makes a complete, self-sufficient claim. If it doesn't, it needs to be joined to the sentence before or after it.

• Which is why so many families are moving to the suburbs.(dependent clause with no independent clause attached)
• House prices in the city centre have doubled, which is why so many families are moving to the suburbs.

There's no official quota, but a well-graded Band 7 essay typically has at least one complex sentence in every body paragraph, often two or three, mixed deliberately with shorter simple sentences for readability. An essay built entirely from complex sentences reads as dense and can start introducing more errors per sentence than a mixed style would.

A useful habit is to plan one complex sentence per paragraph before you start writing — usually the sentence that states your main claim or explains a cause-effect link — and let the supporting sentences stay simpler. This keeps range visible without sacrificing accuracy.

In real-time speech, complex sentences are harder to plan and finish cleanly than in writing, so examiners don't expect the same density as a Task 2 essay. One or two well-controlled complex sentences per answer — especially in Part 3, where you're asked to reason and generalise — demonstrate range far more convincingly than a long sentence that loses its own structure halfway through.

If you feel a complex sentence collapsing as you speak it, it is completely acceptable to stop, restart, or simplify — a short self-correction reads as normal spoken English, while pushing through a broken structure produces exactly the kind of error examiners are trained to notice. Building this instinct in practice, before the real exam, is far safer than discovering it mid-interview.