Relative Pronouns

Subject	Object	Possessive
who	who(m)	whose
which	which	whose
that	that

•          We use who and whom for people, and which for things.

Or we can use that for people or things.

•           We use relative pronouns:

- after a noun, to make it clear which person or thing we are talking about:

the house that Jack built

the woman who discovered radium

an eight-year-old boy who attempted to rob a sweet shop

- to tell us more about a person or thing:

My mother, who was born overseas, has always been a great traveller.

Lord Thompson, who is 76, has just retired.

We had fish and chips, which is my favourite meal.

•        But we do not use that as a subject in this kind of relative clause.
•        We use whose as the possessive form of who:

This is George, whose brother went to school with me.

•          We sometimes use whom as the object of a verb or preposition:

This is George, whom you met at our house last year.

This is George’s brother, with whom I went to school.

•           But nowadays we normally use who:

This is George, who you met at our house last year.

This is George’s brother, who I went to school with.

•        When whom or which have a preposition the preposition can come at the beginning of the clause...

I had an uncle in Germany, from who[m] I inherited a bit of money.

We bought a chainsaw, with which we cut up all the wood.

… or at the end of the clause:

I had an uncle in Germany who[m] I inherited a bit of money from.

We bought a chainsaw, which we cut all the wood up with.

•           We can use that at the beginning of the clause:

I had an uncle in Germany that I inherited a bit of money from.

We bought a chainsaw that we cut all the wood up with.

Source: http://learnenglish.britishcouncil.org/es/english-grammar/pronouns/relative-pronouns