Two different definitions are important here.

• Sentences are made up of clauses.
• A clause is an assembly of words that must contain a subject and a verb. 

Global warming, which is defined as the rise in the earth’s temperature, is dangerous. 

The part which is defined as the rise in the earth’s temperature is not a sentence but a clause. It has a subject which, and a verb in the passive form is defined, but it does not carry the full meaning of a sentence. In other words, it is incomplete; therefore, it cannot stand on its own. It has to accompany another part to provide meaning. 

On the other hand, the part that global warming is dangerous is a complete sentence. 

Clauses in the English language are of two sorts: independent and dependent.

An independent clause has a subject and a verb and communicates a complete thought, such as the previous example: Global warming is dangerous. 

And, it is often formed with a complement such as adjectives. 

In contrast, a dependent clause must start with what is called a subordinator. These are words like when, while, if, that, or who. Subordinators do not communicate a complete thought, and if written without an independent clause, they create what is called a fragment. When a dependent clause is added to an independent clause, this creates advanced grammar called complex structure. 

Complex sentences are the most preferable grammatical structure in the IELTS test.

Check out the following excerpts from the IELTS band descriptors of band 7 in speaking, writing task one, and writing task two. (We will discuss this abundantly.)

• Speaking:
  • uses a mix of simple and complex structures, but with limited flexibility
  • may make frequent mistakes with complex structures, though these rarely cause comprehension problems.

• Writing Task One:
  • A variety of complex structures is used with some flexibility and accuracy.

• Writing Task Two:
  • A variety of complex structures is used with some flexibility and accuracy.

Here is a list of some of the subordinators available that are most commonly used. 

after, before, although, even though, if, as soon as, because, so that, that, when, where, which, who, whom, whose, and while

Now, types of sentences in the English language

In the book Writing Academic English, a sentence is characterized as “a group of words that you use to communicate your ideas.”

There are four fundamental sentence structures in English: simple sentences, compound sentences, complex sentences, and compound-complex sentences. 

1. Simple Sentences

A simple sentence is formed by one independent clause that could include

• one subject and one verb: Smartphones can cause psychological issues.
• two subjects and one verb: Smartphones and video games can cause psychological issues.
• one subject and two verbs: Video games can cause psychological issues and inflict physical harm.
• two subjects and two verbs: Smartphones and video games can cause psychological issues and inflict physical harm.

All of these are examples of simple sentences, even though some of them can be long.

2. Compound Sentences

When two independent clauses join together with a coordinator, such as and, this creates a compound structure.

• Video games can cause psychological issues.
• They can inflict physical harm.

The previous two sentences are two independent clauses that could be linked together to create one compound sentence as follows:

Video games can cause psychological issues, and they can inflict physical harm.

There are three different ways to make compound sentences that you can check out here.

3. Complex Sentences

These types of sentences contain one independent clause and at least one dependent clause. 

There are three types of dependent clauses: adverb, adjective, and noun.

• Complex structure with adverb clauses

Adverb clauses behave like adverbs, which means that they tell information about where, when, why, and how something happens. They start with subordinators: when, while, because, although, if, so, or that. Adverb clauses can come before or after an independent clause.

Ex.:  Although there are many causes for global warming, burning fossil fuels remains the most contributing factor to the predicament.

For more on this structure, click here.

• Complex structure with adjective clauses

Adjective clauses behave like adjectives. Adjective clauses describe nouns and usually start with relative pronouns: who, whom, which, whose, or, that, or with relative adverbs: where and when. They follow the noun or pronoun it describes.

Ex.: Global warming, which is more threatening than people think, is caused by burning fossil fuels.

• Complex structure with noun clauses

Noun clauses begin with a wh-question word, that, whether, and sometimes if. It behaves as the noun for the dependent clause; it could either come as a subject or an object.

Ex.: What caused global warming is burning fossil fuels.

4. Compound-Complex Sentences

When we have three clauses, and at least one of them is an independent clause, this is what is called a compound-complex structure. We can create this advanced form by combining any dependent and independent clauses.

Ex.: While there are myriad causes for global warming, the main contributing factor is burning fossil fuels, which remains in use today despite its irreversible damage to the environment. 

Please note that the redundant use of compound-complex sentences while writing for the IELTS could be risky as you may make mistakes in punctuation, which can hurt your score badly. So, do not overuse this structure. And, when you do use it, make sure you have mastered punctuation first.