句，或相当于一个状语从句。
I received a letter from my sister, who is studying in the university. 连接手段
A由关系代词引导：who, whom, whose, that, which等
B由关系副词引导：when，where，why等
1. 关系代词在定语从句中作主语时，从句谓语动词
的人称和数要和先行词保持一致：
Is he the man who/that wants to see you? 2. 关系副词
when,where,why的含义相当于“介词+which”结构： There are occasions when (on which) one must yield(屈服).
3. that 代替关系副词，在口语中省略：
His father died the year (that/when/in which) he was born.
4. 只能用that的情况： 1） 在there be句型中 2） 在不定代词，如
anything, nothing, the one, all, much, few, any, little等：All that is needed is a supply of oil.
3） 先行词有the only, the very 修饰时
4） 先行词为序数词、数词、形容词最高级时：English is the most difficult subject that you will learn during these years.
5） 先行词即有人又有物: Watch the girl and her dog that are crossing the street.
6) 主句已有疑问词who，which：
Which is the bike that you lost?
