什么时候必须用that引导定语从句？

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.