Choosing between ‘which’ and ‘that’ isn’t always intuitive. It all comes down to whether you’re using a defining or non-defining clause. It sounds a bit confusing, it is explained in simpler terms below.
Defining Relative Clauses
Defining relative clauses give essential information about a noun (a person, place or thing).
Look at these sentences:
That is the man who/ that plays for Arsenal.
‘who/ that plays for Arsenal’ is a relative clause. This sentence tells us what man we are talking about. The main clause doesn’t make sense without the relative clause.
This is the dress which/ that I bought in the sales.
‘which/ that I bought in the sales’ is a relative clause. It tell us what skirt we are talking about.
The relative clause follows the word that it defines.
Use who or that to talk about people.
Steve Jobs is the person who/ that co-founded Apple.
Use which or that to talk about things/animals.
The pasta which/ that you cooked yesterday was delicious.
Use where to talk about places.
That’s the café where I usually have dinner.
We use whose to talk about possession by people and animals.
He’s marrying a girl whose family don’t like him.
Who, which and that can be omitted when it is the object of the clause.
Is that the dish (that) I ordered?
In this sentence ‘dish’ is the object of the verb (order). ‘I’ is the subject.
When the relative pronoun is the object, it can be omitted.
The woman (who) he met on the plane was a famous travel blogger.
In this sentence ‘woman’ is the object of the verb (meet). ‘he’ is the subject.
When the relative pronoun is the subject of the clause, it cannot be omitted.
That is my neighbor who has a Labrador.
NOT That is my neighbor has a Labrador.
Where and whose can never be omitted.
He’s marrying a girl whose family don’t like him
NOT He’s marrying a girl family don’t like him.
Non-Defining Relative Clauses
Non-defining relative clauses give extra information that is not important. If this clause is omitted, the meaning of the sentence doesn’t change without the relative clause, although we have less detail.
Look at these sentences:
The dress, which I bought in the sales, only cost £10.
‘which I bought in the sales’ is a non-defining relative clause. It adds extra information. If we take the clause out of the sentence, the sentence still has the same meaning.
My phone, which is only one year old, broke down last night.
‘which is only one year old’ is a non-defining relative clause. It doesn’t tell us which phone we are talking about. It is clear from the sentence which phone is being talked about.
Anna, who was the best student in our group, passed her exam with distinction.
‘who was the best student in our group’ is a non-defining relative clause.
In non-defining relative clauses you cannot omit relative pronouns.
In non-defining relative clauses, you cannot replace other pronouns with that:
NOT The dress, that I bought in the sales, only cost £10.
Use commas to separate a non-defining relative clause from the rest of the sentence.