Module 2 - Chapter 9

Complex Sentence Structures

Master subordinating conjunctions, relative clauses, noun clauses, and adverbial clauses for fluent, academic-level writing.

Introduction

Every sentence you write or speak falls into one of four types: simple, compound, complex, or compound-complex. Understanding these structures is the difference between writing that sounds choppy and immature, and writing that flows with clarity and sophistication.

Building Blocks of Sentences

Think of sentences like buildings. A simple sentence is a single room — functional, but limited. A compound sentence puts two rooms side by side. But a complex sentence builds rooms on top of each other, creating depth and structure. The dependent clause supports the independent clause, just as a foundation supports the floors above it.

Simple Sentence

One independent clause with a subject and verb.

"The rain stopped."

Structure: [Subject + Verb]

Compound Sentence

Two or more independent clauses joined by a coordinating conjunction (FANBOYS).

"The rain stopped, and the sun came out."

Structure: [IC] + conjunction + [IC]

Complex Sentence

One independent clause + one or more dependent clauses.

"Although the rain stopped, the streets were still wet."

Structure: [DC] + [IC] or [IC] + [DC]

Compound-Complex

Two+ independent clauses and at least one dependent clause.

"When the rain stopped, the children went outside and they played in the puddles."

Structure: [DC] + [IC] + conjunction + [IC]

Why Complex Sentences Matter

Fluency: They connect ideas logically instead of producing a string of short, choppy sentences
Precision: They show how ideas relate — cause, contrast, time, condition
Academic readiness: Essays, reports, and formal writing demand complex structures
Speaking naturally: Native speakers use complex sentences constantly in conversation

Key Terminology

Independent clause (IC): A group of words with a subject and verb that can stand alone as a sentence. "She finished the report."
Dependent clause (DC): A group of words with a subject and verb that cannot stand alone. It needs an independent clause. "Because she finished the report..." (What happened? We do not know yet.)
Subordinating conjunction: The word that turns an independent clause into a dependent one (because, although, when, if, etc.).

Subordinating Conjunctions

Subordinating conjunctions are the glue of complex sentences. They introduce a dependent clause and show its relationship to the main clause — whether that relationship is about time, reason, condition, or contrast.

How Subordinating Conjunctions Work

A subordinating conjunction is like a label on a box. The box (dependent clause) contains information, and the label tells you what kind of information it is: a reason? a condition? a time? Without the label, the box just sits there, incomplete.

Reason / Cause

because, since, as

Because it was raining, we cancelled the picnic.
She left early since she had an appointment.
As the deadline was near, everyone worked overtime.

Contrast / Concession

although, though, even though, while, whereas

Although he studied hard, he did not pass.
She accepted the job even though the salary was low.
While I enjoy cooking, I rarely have time for it.

Time

when, while, before, after, until, since, as soon as

When the bell rings, students leave the classroom.
I will wait until you are ready.
Before you leave, please lock the door.
After she graduated, she moved abroad.

Condition

if, unless, provided that, as long as

If you practice every day, you will improve.
You will fail unless you study.
I will help you as long as you are honest with me.

Comma Rule

Dependent clause first? Use a comma.
Because it was late, we went home.

Independent clause first? Usually no comma needed.
We went home because it was late.

Complete List of Common Subordinating Conjunctions (Click to expand) ▼

Conjunction	Relationship	Example
because	reason	I stayed home because I was sick.
although	contrast	Although she tried, she could not solve it.
while	time / contrast	While I was sleeping, the phone rang.
when	time	When I arrive, I will call you.
if	condition	If it rains, we will stay inside.
unless	negative condition	You cannot enter unless you have a ticket.
since	reason / time	Since you asked, I will explain.
until	time	Wait here until I return.
as	reason / time	As the sun set, the sky turned orange.
after	time	After we ate dinner, we took a walk.
before	time	Finish your homework before you watch TV.

Practice: Complete the Sentence

Choose the correct subordinating conjunction for each blank.

_______ the weather improves, the event will be cancelled. (unless / because)
[Show Answer]
_______ she had studied abroad, her English was excellent. (since / until)
[Show Answer]
I will not leave _______ you tell me the truth. (until / although)
[Show Answer]
_______ he apologized, she refused to forgive him. (even though / because)
[Show Answer]

Relative Clauses

Relative clauses modify nouns. They let you combine two sentences into one by using a relative pronoun (who, which, that, whose, whom) or a relative adverb (where, when, why) to connect them.

Relative Pronouns at a Glance

Pronoun	Used For	Example
who	people (subject)	The teacher who teaches math is retiring.
whom	people (object, formal)	The person whom I called did not answer.
which	things / animals	The book, which was published in 2020, became a bestseller.
that	people or things (defining only)	The car that I rented broke down.
whose	possession	The student whose project won received a scholarship.
where	places	The city where I grew up has changed a lot.
when	time	I remember the day when we first met.

Defining vs. Non-Defining Relative Clauses

This is one of the most important distinctions in English grammar. The difference changes both the meaning and the punctuation of a sentence.

Defining (Restrictive)

Essential information. Without it, we do not know which noun is meant. No commas.

The students who studied hard passed.
(Only those students who studied hard — not all students.)
The book that I borrowed was interesting.
(A specific book, not just any book.)
People who exercise regularly are healthier.
(Only those people, not all people.)

Can use: who, which, that

Non-Defining (Non-Restrictive)

Extra information. The noun is already identified. Commas required. Removable.

My brother, who lives in London, is a doctor.
(I have one brother. The clause adds extra info.)
Paris, which is the capital of France, attracts millions of tourists.
(We already know which city.)
My car, which I bought last year, needs repairs.
(I have only one car.)

Can use: who, which (NEVER "that")

Remember: The Comma Test

If you can remove the clause and the sentence still makes sense and identifies the noun clearly, it is non-defining (use commas). If removing it makes the noun vague or changes the meaning, it is defining (no commas).

Practice: Defining or Non-Defining?

Add commas where necessary. If no commas are needed, write "correct as is."

The woman who lives next door is a nurse.
[Show Answer]
London which is the capital of England is very expensive.
[Show Answer]
Students who cheat will be expelled.
[Show Answer]
My mother who is 60 years old still works full-time.
[Show Answer]

Noun Clauses

A noun clause is a dependent clause that functions as a noun. It can be the subject, the object, or the complement of a sentence. Noun clauses often begin with that, what, who, whether, if, how, where, when, or why.

Think of It This Way

In the sentence "I know the answer," the object is a simple noun. But you can replace it with an entire clause: "I know what you mean." The clause "what you mean" does the same job as "the answer" — it is a noun clause acting as the object.

Three Types of Noun Clauses

That-Clauses

Begin with that (often omittable in informal speech).

I believe (that) honesty is important.
That she passed the exam surprised everyone. (subject)
The problem is that we have no budget. (complement)
He told me that he would be late. (object)

Wh-Clauses

Begin with what, who, where, when, why, how.

I do not know what she wants.
Where he went remains a mystery.
Can you explain why this happened?
How they solved the problem was impressive.

Note: The word order in noun clauses is subject + verb, not question order. "I wonder where she is" (not "where is she").

If / Whether Clauses

Used for embedded yes/no questions and uncertainty.

I wonder if she is coming.
Nobody knows whether the plan will work.
Whether we go or stay depends on the weather.
Tell me if you need any help.

Tip: Use "whether" in formal writing. "Whether" works everywhere; "if" does not work as a subject clause.

Noun Clause Functions

Function	Example
Subject	What he said was shocking.
Direct object	I understand why you are upset.
Subject complement	The truth is that nobody cares.
Object of preposition	I am interested in what you think.

Practice: Identify the Noun Clause

Find the noun clause in each sentence and identify its function.

Nobody knows where the treasure is hidden.
[Show Answer]
That he lied was obvious to everyone.
[Show Answer]
The question is whether we can afford it.
[Show Answer]
She is curious about how the machine works.
[Show Answer]

Adverbial Clauses

Adverbial clauses modify verbs, adjectives, or other adverbs. They answer questions like when?, why?, under what condition?, despite what?, and for what purpose? They are introduced by subordinating conjunctions and are the most varied type of dependent clause.

Time Clauses

when, while, before, after, until, as soon as, since

When I was young, I lived in a small village.
I will call you as soon as I arrive.
While she was cooking, the doorbell rang.
He has worked here since he graduated.

Reason Clauses

because, since, as, seeing that, given that

She left early because she was feeling ill.
Since you are not busy, can you help me?
Given that the evidence is strong, the verdict was expected.

Condition Clauses

if, unless, provided that, as long as, in case

If you heat water to 100°C, it boils.
You will not succeed unless you try.
Take an umbrella in case it rains.
I will agree provided that you meet the deadline.

Concession Clauses

although, though, even though, while, whereas

Although the test was difficult, most students passed.
He kept running even though he was exhausted.
Whereas she enjoys fiction, her sister prefers non-fiction.

Purpose Clauses

so that, in order that, so as to

She studied hard so that she could pass the exam.
He whispered so that no one would hear.
They left early in order that they might avoid traffic.

Result Clauses

so...that, such...that

It was so cold that the lake froze.
She is such a talented singer that everyone stops to listen.
He spoke so quickly that nobody understood him.

Quick Reference: so vs. such

so + adjective/adverb + that: It was so hot that we stayed indoors.
such + (a/an) + noun phrase + that: It was such a hot day that we stayed indoors.

Both express the same idea — an extreme quality leading to a result. The difference is grammatical: "so" goes before adjectives/adverbs, "such" goes before noun phrases.

Practice: Identify the Type of Adverbial Clause

I will forgive you if you promise not to do it again.
[Show Answer]
Even though the restaurant was expensive, the food was mediocre.
[Show Answer]
She moved to the front row so that she could hear better.
[Show Answer]
As soon as the meeting ended, everyone rushed out.
[Show Answer]
The movie was so boring that half the audience left.
[Show Answer]

Common Mistakes & Sentence Combining Practice

Knowing how to build complex sentences is only half the battle. You also need to avoid the three most common structural errors that plague writers at every level.

Three Deadly Errors

1. Run-On Sentences

Two independent clauses joined with no punctuation or conjunction.

Wrong: She was tired she went to bed.

Fixed:

She was tired, so she went to bed. (compound)
Because she was tired, she went to bed. (complex)
She was tired. She went to bed. (two simple)

2. Comma Splices

Two independent clauses joined by only a comma (no conjunction).

Wrong: The exam was hard, many students failed.

Fixed:

The exam was hard; many students failed. (semicolon)
The exam was hard, so many students failed. (add conjunction)
Because the exam was hard, many students failed. (complex)

3. Sentence Fragments

A dependent clause presented as a complete sentence.

Wrong: Because she studied hard. (What happened?)

Fixed:

Because she studied hard, she passed the exam.
She passed the exam because she studied hard.

A dependent clause always needs an independent clause to lean on.

Sentence Combining Practice

The best way to master complex sentences is to practice combining simple sentences into more sophisticated ones. Combine each pair using the technique indicated.

Exercise: Combine the Sentences

Use a relative clause: The scientist won the Nobel Prize. She discovered the new element.
[Show Answer]
Use an adverbial clause of time: The alarm went off. Everyone evacuated the building.
[Show Answer]
Use an adverbial clause of concession: The hotel was expensive. We decided to stay there.
[Show Answer]
Use a noun clause: What caused the accident? Nobody knows.
[Show Answer] Nobody knows what caused the accident.
Use a result clause: The music was very loud. We could not hear each other.
[Show Answer]
Use a purpose clause: He saved money. He wanted to buy a car.
[Show Answer]
Use a condition clause: You do not wear a seatbelt. You could get seriously injured.
[Show Answer]
Fix the error: Although she was qualified for the position. She did not get the job.
[Show Answer]

Complex Sentence Structures

Introduction

Building Blocks of Sentences

Simple Sentence

Compound Sentence

Complex Sentence

Compound-Complex

Why Complex Sentences Matter

Key Terminology

Subordinating Conjunctions

How Subordinating Conjunctions Work

Reason / Cause

Contrast / Concession

Time

Condition

Comma Rule

Practice: Complete the Sentence

Relative Clauses

Relative Pronouns at a Glance

Defining vs. Non-Defining Relative Clauses

Defining (Restrictive)

Non-Defining (Non-Restrictive)

Remember: The Comma Test

Practice: Defining or Non-Defining?

Noun Clauses

Think of It This Way

Three Types of Noun Clauses

That-Clauses

Wh-Clauses

If / Whether Clauses

Noun Clause Functions

Practice: Identify the Noun Clause

Adverbial Clauses

Time Clauses

Reason Clauses

Condition Clauses

Concession Clauses

Purpose Clauses

Result Clauses

Quick Reference: so vs. such

Practice: Identify the Type of Adverbial Clause

Common Mistakes & Sentence Combining Practice

Three Deadly Errors

1. Run-On Sentences

2. Comma Splices

3. Sentence Fragments

Sentence Combining Practice

Exercise: Combine the Sentences

Knowledge Check

Quiz Time

Quiz Complete!