[Humble]

Hi, dean1,
It would take quite a while to tell all the rule. Basically, you can use both which and that as conjunctions talking abt inanimate subjects. With people it's either who or that.
But which and thare not always interchangeable. Here is an explanation from the BBC English.
Now, in a relative clause, we can use 'who' or 'whom' for people, and 'which' for things. So we can say: 'the man who came to dinner', or 'the bridge which crosses the Ganges up river from here'. So: 'the man who came to dinner', 'the bridge which crosses the Ganges'.
Now, 'that' is less formal, and it can be used for both people and things in some relative clauses. So I could say, less formally: 'the man that came to dinner', 'the bridge that crosses the Ganges'.
But, 'that' can only be used in what we call identifying relative clauses and those are clauses where you need the information to understand what you're talking about. Those were both identifying relative clauses, but if I said: 'Mr Swan, who came to dinner', I don't need 'who came to dinner' to define Mr Swan, I've already identified him. So, you can not use 'that' in that sentence, and you can not use 'that' if you are talking about: 'Waterloo Bridge, which crosses the Thames up river from here'. So, that's when you use 'which' for identifying relative clauses and for non-identifying relative clauses, but you can only use 'that' informally for identifying relative clauses. Is that clear?

Vaibhav responds:
Can we take certain examples for this, like, there is a group of presidents who are meeting in the conference: 'the president who is from India', 'the president which is from India', 'the president that is from India' - which one is correct?

Catherine Walter replies:
OK - you can't use 'which' for a president, because a president is a person. You can use 'who' or 'that'. If there are several presidents and you want to talk about 'the president that is coming there', instead of 'the president that's not coming there'. But if by saying 'the president' it's clear that you mean only one person, then you can not use 'that'. You have to say 'who': 'the president who is coming to the conference'.
Best wishes,
Humble