Q & A When do we use that instead of who or which?

Basic answer

The who, which and that the question refers to are all relative pronouns.

We can use that instead of who or which in defining (restrictive) relative clauses:

• For people:
• - The boy who is climbing the tree is my son.
• or The boy that is climbing the tree is my son.
• For things:
• - The car which is coming up the hill is my father's.
• or The car that is coming up the hill is my father's.

Note that I said can; you don't have to use that. If you are happier using who and which (which many learners are), by all means do. Just be aware that many native speakers will often use that.

Remind me about the difference between defining and non-defining relative clauses.

Defining relative clauses (aka restrictive relative clauses)

Look at those two sentences again:

• - The boy who (that) is climbing the tree is my son.
• - The car which (that) is coming up the hill is my father's.

What happens when we take the relative clauses away?

• - The boy is my son.
• - The car is my father's.

What boy? What car? We no longer know which boy or which car is being talked about. Those relative clauses tell us which; they identify the noun and are essential to the meaning if the sentence, so are defining.

Non-defining relative clauses (aka non-restrictive relative clauses)

Now look at these sentences:

• - My son, who loves climbing trees, is the one in the blue jumper.
• - That red sports car, which can go incredibly fast, is my father's.

And what happens now when we take the relative clauses away?

• - My son is the one in the blue jumper.
• - That red sports car is my father's.

These sentences make perfect sense. The relative clauses gave us extra non-essential information. They are therefore non-defining.

The use of commas and pauses

As we can use who and which for both defining and non-defining relative clauses, the main way of distinguishing between them (apart from context) is in the use of commas in writing, and pauses when speaking.

In non-defining clauses, so that our listener knows that the information in the relative clause is extra, we pause slightly before and after this extra information when speaking. This is reflected in written English by the use of commas to separate off the non-defining relative clause.

But in defining clauses, because the information is essential to the meaning, there is no reason to pause when speaking. Similarly when writing we don't use commas.

Omitting the relative pronoun altogether in defining relative clauses

In the defining relative clauses we've been discussing, the relative pronoun refers to the subject of the verb that immediately follows it and cannot be left out.

• - The boy who (that) is climbing the tree is my son.
• - The car which (that) is coming up the hill is my father's.

But in the following sentences the relative pronoun refers to the object of the verb that follows it and can be left out.

• For people:
• or The professor who I just mentioned teaches astro-physics.
• or The professor that I just mentioned teaches astro-physics.
• or The professor I just mentioned teaches astro-physics.
• For things:
• - The ladder which he is using is a bit old.
• or The ladder that he is using is a bit old.
• or The ladder he is using is a bit old.

One easy way of telling whether the relative pronoun refers to the object is to see whether there is any noun or pronoun between it and the verb. If there is, then that must be the subject, and so the relative pronoun must refer to the object.

For any traditional grammarian reading that sentence: The professor who I just mentioned teaches astro-physics. and having an apoplexy because I used who and not whom, please see the note below.

Non-defining relative clauses

Just remember that we can't substitute that for who or which in non-defining relative clauses.

• For people:
• - That boy over there,who is climbing the tree, is my son.
• NOT That boy over there,that is climbing the tree is my son.
• For things:
• - That car, which is coming up the hill, is my father's.
• NOT That car, that is coming up the hill, is my father's.
• For the whole idea:
• - They've finally decided to get married, which is wonderful news.
• NOT They've finally decided to get married, that is wonderful news.

And what about whom?

The truth is that most of us never use whom in defining relative clauses. We don't even have to make the choice between who and whom because, as whom is only ever used to refer to the object, the most natural thing is to leave it out altogether, as in the sentences above.

But if you ever have to do any formal written work, especially academic work, you'd probably be safer writing, The professor whom I just mentioned teaches astro-physics, just in case.

Nor is whom often used in non-defining relative clauses, except after expressions like: all of, many of, half of etc., when we must use it.

• My friends, most of whom are married, like to spend the weekend with their families.
• His family, half of whom live abroad, only get together very rarely.

Usage and style notes

Some people seem to believe in one or both of these two rules:

• When talking about things in defining (restrictive) relative clauses, that should always be used instead of which.
• When talking about people in defining (restrictive) relative clauses, who is preferred to that.

Note that these are style rules invented by the writers of style guides and have nothing to do with grammar. As far as grammar (as it is understood in EFL) is concerned you can use that or which, and that or who. And the great writers of English literature such as Jane Austen did exactly that, sometimes using one, sometimes using the other, apparently as the fancy took them.