Complex sentence: definition and type

Compound sentences are a basic sentence structure in English. What is a compound sentence, and what are some basic structures? How do you differentiate compound sentences, complex sentences, and simple sentences? Let's learn together about the different types of compound sentences in English in the article below!

I. Complex sentence definition 

Complex sentences in English refer to a sentence structure that contains one independent clause and one or more dependent clauses. In this structure, the clauses are connected by subordinate conjunctions or relative pronouns. Below are some complex sentence examples:

• An independent clause can stand alone as a complete sentence. 
• A dependent clause cannot stand alone as a complete sentence and must be attached to an independent clause.

For example:

• Because the test was too difficult, I got a bad grade. ➡ The dependent clause is Because the test was too difficult, the independent clause is  I got a bad grade. 
• Because it rained a lot, the tree was waterlogged. ➡ The dependent clause is Because it rained a lot, the independent clause is the tree was waterlogged. 

II. Types of complex sentences in English 

How many common types of complex sentences are there? Below are the most common types of complex sentences in English. Let’s check them out!

1. Complex sentences use subordinating conjunctions 

Complex sentences in English that use subordinating conjunctions have the following main forms:

1.1. Cause and effect relationship

Complex sentences in English expressing cause-and-effect relationships have the following structure:

As/since/because + clause, clause.

Or

Clause + because of/due to + noun/gerund.

For example:

• As social networks are developed, we can connect with each other more easily. 
• A lot of animals have died because of forest fires. 

In cases expressing cause and effect, it is followed by a noun/noun phrase with the following structure:

Because of/ Due to/ Owing to + noun/ gerund, clause.

Or

Clause + because of/ due to/ owing to/ thanks to + noun/ gerund.

For example: 

• Because of the rain, the trees have grown faster. 
• I have successfully started my business thanks to the support of my family. 

1.2. Concession relationship

Complex sentences in English expressing a concession relationship have the following structure:

Although/ Though/ Even though + clause, clause.

Or

Clause + although/ though/ even though + clause.

For example: 

• Even though today is my day off, I’m still working. 
• I still love him although he treats me badly. 

1.3. Contrast relationship

Complex sentences in English express contrasting relationships using subordinating conjunctions with the following structure:

While + clause, clause. 

Clause + while/whereas + clause. 

Whereas clause, clause.

For example:

• While my mother prefers to live in the countryside, my father prefers the city. 
• He fell asleep while everyone was working hard. 
• Whereas I like staying at home, my boyfriend likes to go out. 

1.4. Purposes

Complex sentences in English express purpose using subordinating conjunctions with the following structure:

Clause + in order that/ so that + clause (modal verb + a bare infinitive verb). 

For example: 

• I always turn off the lights before bed so that I can fall asleep easily. 
• She wakes up early in order that she can catch the bus. 

1.5. Time

Complex sentences in English that express time using subordinating conjunctions have the following structure:

Adverb of time + clause, clause.

Or

Clause + adverb of time + clause.

Some common time adverbs are: before, after, since, until, etc... For example:

• After I graduate from university, I will go back to my hometown to work. 
• You should turn off the lights before you leave your room.

1.6. Hypotheses

In the case of complex sentences in English used to express hypotheses, the structure is as follows:

If/As long as/Unless/In case + clause, clause.

Or

Clause + if/as long as/unless/in case + clause.

For example: 

• If you want to do well on the test, you should study hard. 
• You should check your mailbox regularly in case there is important news.  

In this hypothetical form, the verbs are conjugated in each clause in a similar way to conditional sentences. You can refer to more details about conditional sentences in the following article: "What are conditional sentences? - Concepts, formulas, exercises."

2. Complex sentences use relative clauses

Complex sentences in English using relative clauses have the following basic structures:

III. The formation of a complete complex sentence

To construct complete complex sentences in English, you can apply the formulas mentioned in Part II, while also paying attention to the use of commas in sentences. If the independent clause comes before the dependent clause, a comma is not necessary. However, if the independent clause comes after the dependent clause, a comma should be used. Here's an example of constructing complex sentences in English:

• Because it was raining, the camping trip was canceled. In this sentence, the independent clause comes after, so a comma is needed.
• He got sick because he walked in the rain. In this sentence, the independent clause comes before, so no comma is needed.

IV. The differentiation of single sentences, compound sentences, and complex sentences in English 

To distinguish complex sentences in English from simple and compound sentences, we rely on the concepts and structures, as follows:

V. Some notes on complex sentences in IELTS Writing

Using complex sentences appropriately in the IELTS Writing test will help you improve your band score and be highly evaluated in the Grammatical Range and Accuracy criterion (the ability to use flexible and diverse grammar structures). To effectively use complex sentences, prioritize using them when presenting the main arguments in your essay, usually placed at the beginning of paragraphs. Additionally, candidates can interweave complex sentences in the development of ideas to support their arguments.

Please note that you should not overuse complex sentences but rather find the right balance between simple, compound, and complex sentences.

Here's an example of how you can use a complex sentence to express a disagreement about using phones at night: We should not use the phone at night because it greatly affects our vision and nervous system. 

VI. Exercises on complex sentences with detailed answers

Below are some practice exercises on complex sentences:

Exercise 1: Choose the correct answer

1. She almost missed the train ___________ she has been waiting at the pier for more than 30 minutes. 

• A. although
• B. since
• C. despite
• D. because of

2. I did not eat dinner _____________ I was too tired. 

• A. so
• B. because
• C. but
• D. if

3. We were driving to a village _____________ we caught an accident.

• A. when
• B. while
• C. before
• D. as soon as

4. I want to be a doctor ______ I can save people.

• A. in order to
• B. so as not to
• C. so that
• D. as soon as

5. ___________leaving the house, turn off the lights.

• A. As soon as
• B. Although
• C. After
• D. Before

Exercise 2: Rewrite the following sentences:

1. I was born in a craft village. I know the process of making pottery. (BECAUSE)

➡ __________________________________________________

2. She was not happy at the party. She wanted to leave. (AS)

➡ __________________________________________________

3. I am sick. I still have to go to work. (ALTHOUGH)

➡ __________________________________________________

4. Tomorrow is a holiday. I have a day off work. (SO)

➡ __________________________________________________

5. I left my job. My boss didn’t give me a raise. (BECAUSE) 

➡ __________________________________________________

Answer keys:

This article has provided useful insight into complex sentences that students should grasp. Please practice them extensively, especially in writing, to enhance your English language proficiency.