我收到的那封信是我父亲写给我的。

在下面几种情况下必须用"that"引导定语从句：

1）先行词是不定代词：all，few，little，much，something，nothing，anything等。例如：

All that we have to do is to practice every day. 我们要做的就是每天练习。

2）先行词是序数词或被序数词修饰，只用that，不用which。例如：

The first lesson that I learned will never be forgotten. 我永远也忘不了所学的第一课。

The train is the last that will go to Suzhou.这是开往苏州的最后一趟车。

3）先行词是形容词最高级或被被形容词最高级修饰，只用that，不用which。

　This is the best that has been used against pollution. 这是用来预防污染的最佳办法。

　This is the most interesting film that I've ever seen. 这是我看过的最有趣的电影。

4）先行词被all, any, every, each, few, little, no, some等修饰。例如：

I have read all the books (that) you gave me.

你给我的书我全读完了。

5）先行词被the only，the very，the last修饰时。

He is the only person that I want to talk to.

他是惟一的我想跟说话的人。

The only possessions that I could see were a few tin plates and cups and a couple of pots.

我能看到的他们仅有的财产就是几个锡盘子、杯子和三两个罐子。

6）先行词既有人又有物时。例如：

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

他们谈论着他们能记起的那些在学校的人和事。

7)当先行词前面有who, which 等疑问代词时。

Who is the boy that won the gold medal?

8) 当有两个定语从句时，其中一个关系代词已经用了which, 另一个宜用that.

Thay secretly built up a small favtory, which produced things that could cause pollution.