Who, that and which are all relative pronouns which are used in a sentence or clause to specify which person or thing or which type of person or thing we are talking about.
Who vs. that
In a defining relative clause, the relative pronoun can be the subject or the object of the clause. When in a sentence, the relative pronoun is the subject of the clause, the verb follows the relative pronoun:
The old woman who lives next to us looked very queer, with witchy hair and soiled clothes.
The man who gave me a stern look will be staying with us in the vacations.
I have a friend who has played in county cricket.
There have been many complaints about the people littering on the sidewalk who/ that live in those flats.
Wilsons have a relative who/ that every so often visits them.
The thought of seeing her after so long was all that kept him awake all night.
Don’t put a comma between the noun and a defining relative clause. Relative clauses begin with a relative pronoun ‘who’, ‘which’ or ‘that’. However, we often drop the wh-word. Instead, a zero relative pronoun is used:
She was probably the most charming woman (that) I have ever worked with.
Martha, the woman (that) Bob married last year, has broken up with him.
We flew to Hong Kong by the airline (which/ that) Pamela had recommended to us.
When the relative pronoun is the object in a sentence, a noun (or pronoun) comes between the relative pronoun and the verb in relative clause. Here, we use a zero relative pronoun:
That’s the man (who/ that) I saw in the police van this morning.
She gifted the necktie to her husband (which/ that) she had brought back from Switzerland.
That vs. which
So, now we move on to the main distinction between that and which, where there’s a slight difference between UK and US English. In UK English, if a clause contains some necessary information about the noun that comes before it, it’s OK to use either that or which:
She took the train which runs between Kansas City and St. Louis. √
She took the train that runs between Kansas City and St. Louis. √
Jack boarded the train that came first.
However, in US English, most Grammar guides recommend that for introducing a restrictive relative clause, use that rather than which:
She gave her all woollens to charity that she bought only two months ago.
We use that as subject after something and anything. Words such as little, much, all and none are used as nouns and superlatives. That or zero relative pronoun is used as subject after these:
That’s all (that) I wanted to explain her. √
That’s all which I wanted to explain her. ✗
Is there anything (that) you need to know about the man? √
Is there anything which you need to know about the man? ✗
That’s the most fascinating place (that) I had ever gone. √
That’s the most fascinating place which I had ever gone. ✗