Relative clauses are subordinate clauses which begin with relative pronouns and are used to tell more about persons or things. Consider the following sentence: This is the woman “whose son died in a plane crash”. The relative clause in this sentence is “whose son died in a plane crash.” As you can see, this relative clause tells us more about the woman in question. Obviously, a relative clause is an adjective clause: it answers the question “which one?” This is the woman… Which woman? The one “whose son died in a plane crash.”

Relative clauses can be introduced by any of the following relative pronouns: who, whom, which, that and whose. Examples:

Who: Jack is the boy who broke the windscreen.

Whom: This is the girl whom Jack raped.

Which: The bag, which I bought last week, has been stolen.

That: Samson was the man that killed a lion with his bare hands.

Whose: He is the doctor whose clinic was demolished.

There are two types of relative clauses: 1. Defining Relative Clause, and 2. Non-defining Relative Clause.

Defining Relative Clause: This type is used to indicate, specify or clarify which person or thing we are talking about. Examples: A. Adichie is the woman who wrote this novel. B. This is the car which Sam bought. In each case, the relative clause directly identifies the person or thing being talked about. We can use “that” instead of who or which. A. Adichie is the woman that wrote this novel. B. This is the car that Sam bought. In example B, we can even leave out “that” completely: This is the car Sam bought. The information that a defining relative clause provides is part of the core meaning of the entire sentence. The information is essential to the meaning of the sentence, such that removing it renders it incomplete. For example, if you say Adichie is the woman, it raises the question: which woman? So, the defining relative clause provides the essential information and defines the identity of the person or thing being talked about.

Non-defining Relative Clause: This type of relative clause merely gives additional or extra information about the person or thing being talked about. It is called a non-essential relative clause because the information it gives is part of the core meaning of the sentence. Examples: A. My uncle, who lives in Warri, is a doctor. As you can see here, the non-defining relative clause is separated with commas. This shows that the information it gives is secondary or additional. The sentence implies this: My uncle is a doctor; by the way, he lives in Warri.

For more detailed audiovisual information:
https://www.youtube.com/watch?v=rkxsThyamdU