Complex Sentences

A complex sentence contains a main clause (an independent idea that can stand alone) and one or more subordinate clauses (dependent ideas that add extra information). At P6, you will bring together everything you know about simple, compound, and complex sentences and learn to combine them confidently.

What You'll Learn

How to identify and construct simple, compound, complex, and compound-complex sentences
How to choose the right sentence structure for different purposes such as adding detail, showing cause and effect, or contrasting ideas
How to combine and rearrange clauses to improve clarity and flow in your writing
How to avoid common errors when mixing multiple clause types in one sentence

When to Use

Adding background information: "The students, who had been revising all week, performed well in the examination."
Showing cause and effect: "Because the monsoon season arrived early, many outdoor events were cancelled."
Contrasting two ideas: "Although the team trained hard, they did not win the tournament."
Expressing time relationships: "The crowd cheered loudly when the fireworks lit up the sky over Marina Bay."
Combining multiple layers of meaning: "She finished her homework before dinner, and after she ate, she practised the piano until her mother told her to stop."

How to Form

The Four Sentence Structures

Structure	What It Contains	Example
Simple	One independent clause	The hawker centre was crowded.
Compound	Two or more independent clauses joined by a coordinating conjunction	The hawker centre was crowded, but we found a table.
Complex	One independent clause + one or more subordinate clauses	Although the hawker centre was crowded, we found a table.
Compound-complex	Two or more independent clauses + at least one subordinate clause	Although the hawker centre was crowded, we found a table, and we ordered chicken rice before the stall closed.

Types of Subordinate Clauses

Clause Type	Introduced By	Function	Example
Adverbial clause	because, although, when, while, if, before, after, until, since, unless, so that	Tells why, when, how, or under what condition	Because it rained, the match was postponed.
Relative clause (adjective clause)	who, whom, whose, which, that	Describes a noun	The boy who sits beside me is my best friend.
Noun clause	that, what, whether, how, why	Acts as a subject or object	I believe that hard work pays off.

Building Compound-Complex Sentences

Start with two simple ideas and a supporting detail:

Simple sentences: "The library was quiet." + "I could concentrate." + "There were many students."
Compound: "The library was quiet, and I could concentrate."
Complex: "Because the library was quiet, I could concentrate."
Compound-complex: "Although there were many students, the library was quiet, and I could concentrate."

Key Rules

Every sentence needs at least one independent clause: A subordinate clause cannot stand alone as a sentence. "Because it was raining." is a fragment, not a sentence.
Use a comma after a subordinate clause that starts a sentence: "When the bell rang, the children lined up." But no comma is needed when the subordinate clause comes after the main clause: "The children lined up when the bell rang."
Choose the right subordinating conjunction for your meaning: Use "because" for reason, "although/even though" for contrast, "when/while/after/before" for time, "if/unless" for condition, and "so that" for purpose.
Relative clauses follow the noun they describe: Place a relative clause directly after the noun it modifies. "The teacher who taught us Science retired last year." Not: "The teacher retired last year who taught us Science."
Use commas around non-defining relative clauses: A clause that gives extra (non-essential) information is set off with commas. "Mr Tan, who is our form teacher, organised the camp." Remove the commas only when the clause is essential to identify the noun: "The student who broke the window must pay for it."
Do not overload a sentence with too many clauses: If a sentence becomes hard to follow, split it into two. Clear writing is more important than long sentences.
Keep the subject and verb of the main clause close together: Inserting too many words between them makes the sentence confusing. Instead of "The girl, after she finished her project, which her teacher had assigned the previous week, went home," write "After the girl finished the project that her teacher had assigned the previous week, she went home."

Common Mistakes

Wrong	Right	Why
Because it was late. I went home.	Because it was late, I went home.	The subordinate clause cannot stand alone; join it to the main clause with a comma
The boy who is my classmate, is very tall.	The boy who is my classmate is very tall.	A defining relative clause does not take a comma before the main verb
Although she studied hard but she failed.	Although she studied hard, she failed.	Do not pair "although" with "but" -- one subordinator is enough
I will go if you will come with me.	I will go if you come with me.	After "if" (conditional), use the simple present, not "will"
The cake that it was on the table is gone.	The cake that was on the table is gone.	The relative pronoun "that" already replaces the subject; do not add an extra "it"
She asked me that where did I go.	She asked me where I went.	In reported questions (noun clauses), use statement word order and remove "did"

Clue Words

Subordinating conjunctions (start adverbial clauses)

because, since, as, although, even though, though, while, when, whenever, before, after, until, till, if, unless, so that, in order that, as long as, provided that

Relative pronouns (start relative clauses)

who, whom, whose, which, that

Noun-clause starters

that, what, whatever, whether, how, why, where, when

Coordinating conjunctions (join independent clauses in compound sentences)

for, and, nor, but, or, yet, so (remember: FANBOYS)

Tip: If you can remove the clause and the sentence still makes sense, it is a subordinate clause. The part that can stand alone is the main clause.

Practice Tips

Clause-spotting exercise: Take a paragraph from your English textbook and underline every subordinate clause. Label each one as adverbial, relative, or noun clause. This trains your eye to recognise sentence structures quickly.
Sentence-combining drill: Start with three simple sentences and combine them into one compound-complex sentence. Then rewrite the same ideas using a different sentence structure. Compare which version sounds clearer.
The comma check: After writing a complex sentence, read it aloud. If you naturally pause after the opening clause, you probably need a comma there. If the subordinate clause comes at the end, check whether a comma is truly needed (usually it is not).
The "stand-alone" test: Cover the subordinate clause with your hand. Does the remaining part still form a complete sentence? If yes, you have identified the main clause correctly. If no, rethink your sentence structure.

Quick Reference

Sentence Type	Structure	Conjunctions Used	Example
Simple	1 independent clause	none	She read a book.
Compound	2+ independent clauses	FANBOYS (for, and, nor, but, or, yet, so)	She read a book, and he played chess.
Complex	1 independent + 1+ subordinate clause	because, although, when, who, which, that, etc.	She read a book because she had free time.
Compound-complex	2+ independent + 1+ subordinate clause	FANBOYS + subordinators	Although she was tired, she read a book, and he played chess.

Subordinate Clause Type	Question It Answers	Key Words	Example
Adverbial (time)	When?	when, while, before, after, until	Before the concert started, we found our seats.
Adverbial (reason)	Why?	because, since, as	We stayed indoors because the haze was severe.
Adverbial (contrast)	Despite what?	although, even though, though	Although he practised daily, he did not make the school team.
Adverbial (condition)	Under what condition?	if, unless, provided that	You may go out if you finish your revision.
Adverbial (purpose)	For what purpose?	so that, in order that	She spoke clearly so that everyone could hear.
Relative (defining)	Which one?	who, which, that	The classmate who lent me the book is from 6A.
Relative (non-defining)	Extra info	who, which (with commas)	Sentosa, which is a popular island, attracts many tourists.
Noun clause	What? / That...	that, what, whether, how	I know that she will do well.

Complex Sentences (P6) (Primary 6)