Relative Pronouns link a relative clause to another part of the sentence. They introduce the relative clause. You can also use the relative pronouns to ask questions, to add some information or identify things or people.
The most common relative pronouns are:
When refering to people use WHO or THAT:
This is the man that got married last week. or This is the man who got married last week.
This is the girl that plays the guitar very well. or This is the girl who plays the guitar very well.
That is the boy that caused the accident. or That is the boy who caused the accident.
Never use THAT between comma.
My sister’s friend, who I met yesterday, is very intelligent.
In this case, the only option is to use ‘who’ because you can never use THAT between comma.
When talking about people use WHOM after the preposition:
She is the woman with whom he went to the wedding.
In this case you have to use whom because of the preposition WITH. See another example:
He is the only person in whom I trust. ‘In’ is a preposition. That’s why it’s necessary to use Whom.
When talking about animals or a things use WHICH or THAT:
The cat that I have is beautiful. or The cat which I have is beautiful.
Those letters that I received are special. or Those letters which I received are special.
The CD that I bought is excellent. or The CD which I bought is excellent.
Again! Don’t use ‘that’ between comma. Example:
This house, which I bought last spring, is very big. In this case it would be wrong to use ‘that’ because of the comma.
When indicating possession by people and animals use WHOSE:
The man whose wife works for me is Mr. White.
This is the book whose characters are strong.
The girl whose dress is pink is Laura.
Also learn from Captain English about Gender.