The relative pronouns are:
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.