Before we answer this question, let's speak about what 'that' and 'which' are from the grammatical point of view. This will help us understand how we should use them in a sentence correctly.
So, 'that' and 'which' are relative pronouns and we use them in relative clauses. Let us remind you that a sentence can consist of a main clause and a subordinate clause. The last one can be:
- a noun clause
- a relative clause
- an adverbial clause
Relative clauses can be introduced by relative adverbs (why, where, when) or by relative pronouns (that, which, who, whom, whose).
For example👇
| Main clause | Relative clause |
| I saw the woman | who bought my car. |
| I read the book | that you recommended. |
Now let's make it clear when we can use 'that' and 'which' in a sentence. Look at this table attentively 👇
| For people | For things |
| who/that | which/that |
| This is the woman who/that I love. | This is the car which/that I bought. |
You should remember that we can not omit that/which in the examples given above because they are subjects of the verb. Try to say a sentence without them and you will miss the meaning.
But when that/which are used as objects, we can omit them.
For example👇
The girl who/that you saw yesterday was my sister.
The girl you saw yesterday was my sister. (who/that can be omitted)
And the last important thing about the usage of the relative pronouns in a sentence. We do not normally use prepositions before them.
For example👇
The city in which she lives is far from here. (not a usual usage)
The city which she lives in is far away. (usual usage)
The city she lives in is far away. ('which' can be omitted)