'Which' vs 'That' Debate

In this post I would like to talk about restrictive and non-restrictive clauses, about ‘which’ and ‘that’, about when to use a comma and some other such things.

First of all, it is NOT necessary that ‘which’ has to be preceded by a comma. Just because you see a ‘which’ clause without commas, it does not mean the option is wrong.

To understand the uses of ‘which’ and ‘that’, we need to understand defining and non-defining relative clauses.

What is a Relative Clause?

It is the clause that begins with a relative pronoun (who, which, that, whom, whose)! We use relative clauses to clarify which person or thing we are talking about or to add extra information about a noun.

For example:
My father, who is 70, goes running every day.
My youngest son, whose work takes him all over the world, is coming home tomorrow.
My son who works for a consultancy is coming home tomorrow.
I’m going to wear the shirt that I bought in Paris.

The relative clauses have been underlined. Note that some are surrounded by commas and some are not.
The ones that are not surrounded by commas clarify which person or thing we are talking about. These are defining relative clauses.
The ones that are surrounded by commas provide extra information about a noun. They are called non-defining relative clauses.

Defining relative clauses:

They define the noun. What do we mean by that? Let’s see.

Example: My son who works for a consultancy is coming home tomorrow.
‘My son who works for a consultancy’ implies that I probably have more than one son and one of them works for a consultancy. He is the one who is coming.

Example: I’m going to wear the shirt that I bought in Paris.
‘the shirt that I bought in Paris’ defines the shirt. I have many shirts but I am going to wear the one I bought in Paris.

- Defining relative clauses can begin with ‘who’, ‘which’ or ‘that’. You use ‘who’ or ‘that’ for people and ‘which’ or ‘that’ for things.
For example: All the sentences given below are correct.
My son who works for a consultancy is coming home tomorrow.
I’m going to wear the shirt which I bought in Paris.
I’m going to wear the shirt that I bought in Paris.

- Also, sometimes you can omit the relative pronoun of defining relative clauses. When the relative pronoun acts as the object of the relative clause, you can omit the relative pronoun.

Example: I’m going to wear the shirt I bought in Paris.- Correct
‘shirt’ here is the object of the verb ‘bought’. The relative clause is ‘I bought the shirt in Paris.’ The relative pronoun replaces ‘the shirt’ which is the object of this clause.
When the relative pronoun is the subject of the relative clause, you cannot omit it.

Example: My son who works for a consultancy is coming home tomorrow. – Incorrect to omit who
‘who’ is the subject of the verb ‘works’. You cannot omit the relative pronoun here.

Non-defining Relative Clauses:

They provide extra information about the noun. In these cases, we already know the person/thing we are talking about.

Example: My father, who is 70, goes running every day.
‘My father’ clearly talks about my father (who we assume is unique). ‘who is 70’ only gives us more information about my father.
Example: My youngest son, whose work takes him all over the world, is coming home tomorrow.
‘My youngest son’ already clarifies that we are talking about my youngest son. ‘whose work…’ only tells us more about him.

- Non defining relative clauses can use most relative pronouns but they cannot use ‘that’. Also, you cannot omit the pronoun.
My father, that is 70, goes running every day. – Incorrect. Cannot use ‘that’
My father, is 70, goes running every day. – Incorrect. Cannot work without the pronoun

I will soon put up some OG questions involving defining and non defining relative clauses.