Who : it is a subject pronoun and we use it when we talk about people. This is the guy who teaches me English.
Whom: it is an object pronoun and we also use it when we talk about people. With whom did you come ? From whom did you take it? Whose: use it when you talk about a possession. Whose pen is this? This is the guy whose father teaches Physics.
Which: we use which if we talk about animals and things. We can use as a subject or object pronoun. Can you see that dog which is playing with a ball.
That: you can use it as a subject or object pronoun when you talk about people and things. He picked up the book that is on the shelf.
Second: Relative adverbs Relative adverbs are: when, where and why.
When: we use it when we refer to a time expression . I recall the day when I had my first car.
Where: we use it when we refer to a place. This the cafe where I go to every day.
Why: we use it when we mention a reason. This is the reason why i'm here tonight.