A complex sentence contains one main clause and at least one subordinate clause, linked by a subordinating conjunction such as 'because', 'although' or 'when'. Unlike a simple sentence, it shows how ideas relate to each other, making it one of the most reliable markers of sophisticated writing in KS3 English.
What is the difference between a main clause and a subordinate clause?
A main clause is a group of words that contains a subject and a verb and makes complete sense on its own. "The dog barked" is a main clause — it can stand as a sentence.
A subordinate clause also contains a subject and a verb, but it cannot stand alone; it depends on the main clause for its full meaning. "Because it heard a noise" is a subordinate clause — it raises the question "because it heard a noise, what happened?" and needs the main clause to complete the thought.
Put them together and you have a complex sentence: "The dog barked because it heard a noise."
The subordinate clause adds information — in this case, the reason — that the main clause alone cannot provide.
What are subordinating conjunctions and how do I use them?
A subordinating conjunction is the joining word that introduces a subordinate clause and signals its relationship to the main clause. The most useful ones to know are:
| Subordinating conjunction | Relationship it signals |
|---|---|
| because, since, as | Cause or reason |
| although, even though, despite the fact that | Contrast or concession |
| when, whenever, while, as | Time |
| if, unless, provided that | Condition |
| so that, in order that | Purpose |
| until, before, after | Sequence in time |
Notice that the subordinate clause can come at the start or the end of the sentence:
- "Although it was raining, they decided to walk." (subordinate clause first)
- "They decided to walk, although it was raining." (main clause first)
When the subordinate clause comes first, it is followed by a comma. When it comes second, no comma is usually needed (though usage varies). Placing the subordinate clause at the start creates a brief build-up before the main point; placing it at the end lets the reader reach the main point quickly before adding the qualification.
What are the three main types of subordinate clause?
Not all subordinate clauses are the same. At KS3, it is useful to recognise the three most common types:
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Adverbial clauses — these answer questions like when, why, how or under what conditions. Most of the examples above are adverbial: "because it heard a noise" explains why.
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Relative clauses — these give extra information about a noun, introduced by a relative pronoun such as who, which, that or whose. Example: "The dog, which had been restless all evening, finally settled." The relative clause adds detail about the dog.
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Noun clauses — these act as the subject or object of the main clause. Example: "I believe that the answer is obvious." Here, "that the answer is obvious" is the noun clause acting as the object of "I believe."
For most KS3 writing tasks, adverbial and relative clauses are the most important to practise.
How do complex sentences improve my writing?
Using only simple sentences creates a choppy, list-like effect that feels unsophisticated. Complex sentences allow you to:
- Show cause and effect: "Sales fell because demand had shifted."
- Show contrast: "Although he had revised carefully, the questions surprised him."
- Qualify a statement: "The character is sympathetic, even though his actions are cruel."
- Add specific detail: "The street, which had been busy an hour earlier, was now deserted."
In English Language writing tasks, the mark scheme for AO6 (accurate and varied sentence structure) directly rewards the deliberate use of different sentence forms, including complex sentences. Varying between simple, compound and complex sentences — rather than using only one type — is one of the most concrete improvements you can make.
What are the most common mistakes when writing complex sentences?
1. Using 'which' when 'that' is needed (or vice versa). Restrictive relative clauses (essential to the meaning) normally use 'that' with no comma: "The book that she recommended was brilliant." Non-restrictive relative clauses (additional, removable information) use 'which' with commas: "The book, which she had recommended, was brilliant."
2. Dangling subordinate clauses. "Although exhausted, the homework was finished." — Who was exhausted? The homework? The subject of the main clause must match the implied subject of the subordinate clause. Fix: "Although exhausted, she finished the homework."
3. Subordinating an important idea. If the most important idea in your sentence is the one that should be emphasised, put it in the main clause, not the subordinate one. A subordinate clause by definition receives less weight.
4. Over-nesting clauses. One subordinate clause per sentence is usually enough. Piling up subordinate clauses creates confusion rather than sophistication.
How does a complex sentence differ from a compound sentence?
A compound sentence joins two or more independent main clauses with a coordinating conjunction ('and', 'but', 'so', 'yet', 'or') or a semicolon. Both halves are equal in grammatical weight:
- "She studied hard, but she still felt nervous." (compound)
- "Although she studied hard, she still felt nervous." (complex)
In the compound version, both clauses are independent and balanced. In the complex version, the 'although' clause is subordinate — it sets up a contrast, but the main weight falls on "she still felt nervous." The choice between compound and complex is a choice about emphasis.
Frequently asked questions
Can a complex sentence have more than one subordinate clause?
Yes. "Although she was tired, she continued working because the deadline was close." contains two subordinate clauses — one of contrast ('although she was tired') and one of cause ('because the deadline was close'). Both are subordinated to the main clause 'she continued working'. Use this construction purposefully; more than two subordinate clauses in a single sentence usually becomes difficult to read.
Is a long sentence always a complex sentence?
No. A sentence can be long and still be simple if it contains only one main clause extended by phrases (not clauses with their own subject and verb). "The old man at the end of the street walked slowly past the broken gate every morning." is a long simple sentence — there is only one verb ('walked') and one subject ('the old man'). Complexity refers to grammatical structure, not length.
How do I identify subordinating conjunctions in an exam?
Ask whether the conjunction could start the sentence without changing the word order. Subordinating conjunctions can: "Because he was late, the train left without him." Coordinating conjunctions ('and', 'but', 'so') cannot move to the front of the combined sentence in the same way.
Do examiners really notice my sentence structures?
They notice the effect your sentences create. Varied, purposeful sentence structure — moving fluently between simple, compound and complex forms — creates rhythm, emphasis and clarity. An essay written entirely in one sentence type feels monotonous, and examiners will comment on it. The goal is not to produce complex sentences for their own sake, but to choose the structure that best serves your meaning at each moment.
Ask Professor Quill to work through your writing with you at aitutors.me — he will help you see where complex sentences would strengthen your argument.