Hello, I am Vaibhav. I am calling from India, and my question is: When do we use 'which' and when do we use 'that'?
What are the constraints, what are the conditions under which we use these two words?
OK - that's a good question.
I'm assuming that you mean in what we call relative clauses since this is where the confusion usually occurs.
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'.
就是这样：'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'.
因此我可以不太正式的说：'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.
那么我就不需要用'who came to dinner'来明确说明Swan先生，因为我已经点出他了。
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'.
还有，当你这么说的时候：'Waterloo Bridge, which crosses the Thames up river from here'，你也不能用that。
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?
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?
'the president who is from India'、'the president which is from India'和'the president that is from India'，这三种哪一个是正确的？
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'.
你必须要说'who'：也就是'the president who is coming to the conference'。
OK. That's clear. That's a great question. Thank you very much.