That: A word with a number of different uses:
1.Demonstrative Pronoun:
I could get on with a man like that.
2.Determiner:
That book is still funny.
3.Relative Pronoun:
I stared back blandly with the same expression that he was trying on me.
4.Subordinating Conjunction:
A couple of times she had complained that he was trying to adopt her.
Which: which is only used to refer to the number of persons, things, or events mentioned or implied?: which of the men answered? which do you want?
Which can be used in a restrictive clause [the war which had just ended, the class to which he spoke], in a restrictive clause preceded by the pronoun that[he sacrificed that which he valued most], in a nonrestrictive clause [my car, which is not running; my family, in which she found a warm welcome], or, archaically, of a person [Our Father, which art in heaven].