1.当先行词是指物的all， little' few， much, any, anything, everything, nothing, none, the one时，或先行词被all， little, few, much, any, every,no等修饰时。如：
All that can be done has been done.
He will tell you everything that he heard about it.
There is little work that is fit for you.
I have eaten all the food that is left.
【注】当先行词是指人的all ， any ， few, one (s) ， anyone ， everyone ， those ， people ,he等时，只能用who 。如：
None so blind as those who won't see.
Anyone who is over sixteen is allowed in.
I met someone who said he knew you.
He who has a mind to beat his dog will easily find his stick.
2.先行词是序数词或最高級形容词时，或者先行词被序数词（包括last，next)、最高級形容词及the only, the very等修饰时。如：
This is the first film that I have seen since I came here.
This is the best that has been used against pollution.
This is the very book that I am looking for.
They often talk about the people and the things that they are interested in.
Luxun and his works that we are familiar with are considered great.
Which is the house that caught fire last night?
Who is the lady that is playing the piano?
This is a book that I have been looking for since last week.