定语从句中连接先行词和定语的叫关系代词或关系副词。有一些情况下，关系代词可以省略：
当关系代词所代替的先行词在定语从句中做宾语，关系词可以省略。Who is the man (that / who / whom) you were talking to? 刚才和你讲话的人是谁？
当关系代词that在定语从句中用作表语时，可以省略。
China is not the country (that) it was. 中国已不是过去的中国了。
关系副词when用于day, year, time少数词后时可以省略（也可换成that）That was the year (that) I first went abroad. 就是那一年我第一次出国。
关系副词where用于place, somewhere, anywhere, everywhere, nowhere少数词后时可以省略(也可换成that)。
This is the place (where) they met yesterday. 这就是他们昨天碰头的地方。
当关系代词that在定语从句中用作宾语补足时，可以省略。
I’m not the madman (that) you thought me. 我并不是你所认为的那个疯子。
关系副词why通常只用于the reason后引导定语从句，且通常可换成that或for which，这些连接词都可以省略。
That’s the reason (why, for which, that) he came. 这就是他来的原因。
Give me one reason (why) we should help you. 给我举出一个我们应当帮助你的理由。