1)先行词是不定代词：
all,few,little,much,something,nothing,anything等.All that we have to do is to practise every day.
2)先行词被序数词或形容词最高级所修饰.
The first lesson that I learned will never be forgotten.
3)先行词被all,any,every,each,few,little,no,some,等修饰.
I have read all the book (that) you gave me.
4)先行词被 the only,the very,the same,the last 修饰时.
He is the only person that I want to talk to.
5)先行词既有人又有物时.
They talked of things and persons that they remembered in the school.
先行词是表示地点时,要根据从句的谓语动词是及物的还是不及物的.如果是及物的就用that(which),否则用where.
This is the house where he lived last year.
This is the house that (which) he visited last year.