Relative pronouns are pronouns that are used to refer to nouns that were mentioned previously. They can be used in essential/defining or non essential clauses/non defining clauses. This means that they can be parenthetical or they can have a main role in making the sentence. When a clause is non-essential/parenthetical, we set it apart using commas.

Relative pronouns
Who – only for people
Whom – only for people
That – for people, animals, or things
Which – for animals or things, or to refer to the whole sentence
Where –  places
When – times
Whose – possession

Students tend to confuse “who and whom;” “that and which;” “whose and who’s.”
Let us be clear.
 
Both who and whom only refer to people, but who refers to subjects and whom refers to objects, mainly objects of a preposition.

Here is the person for whom/to whom/with whom I wrote the letter.
The dentist who pulled out my tooth should be in prison now. – essential
The man, who I hit by mistake, was grumpy and old. – non-essential/parenthetical clause

You can always test that you have made the right choice between WHO and WHOM by substituting with HE and HIM.

WHO = HE
WHOM = HIM
Who/whom took off his clothes at the party?
He/him took off all his clothes at the party.
“Him took off his clothes” is NOT correct.
He took off his clothes is correct.
Therefore, we would say, “Who took off his clothes at the party?

We use whose to show possession.
The man whose heart I stole is now my husband.
Whose is not the same as the contraction who’s. Who’s that? = Who is that?

Many times “that” and “which” are interchangeable. However, we advise that if English is your second language, you concentrate on using “that” when it is essential and saving “which” for non-essential/parenthetical clauses.

For example,
The thesis that I wrote was well received. – essential (no commas necessary)
The thesis idea, which I had to change three times, was well received. – non-essential

The restaurant where we had our first date has just closed down for health violations.

Grandma remembers a time when cell phones did not exist.