什么時候必須用that引導定語從句？

1)先行詞是不定代詞：

all,few,little, much,something,nothing,anything等。All that we have to do is to practise every day.

2)先行詞被序數詞或形容詞最高級所修飾。
The first lesson that I learned will never be forgotten.

3)先行詞被all,any,every, each, few,little,no,some, 等修飾。
I have read all the book (that) you gave me.

4)先行詞被 the only, the very, the same, the last 修飾時。
He is the only person that I want to talk to.

5)先行詞既有人又有物時。

They talked of things and persons that they remembered in the school.

先行詞是表示地點時,要根據從句的謂語動詞是及物的還是不及物的。如果是及物的就用that(which),否則用where。
This is the house where he lived last year.
This is the house that (which) he visited last year.