在阅读文章时，同学们有时会看到这样一种句子现象，句子中没有关系词，却出现了两个不是并列关系的谓语动词。如：I like the music for the reason he dislikes it. 在这个句子中， he dislikes it这是一个定语从句，这个句子实际省略了定语从句的关系词that或是for which。还原成标准的定语从句应该为I like the music for the reason that/for which he dislikes it. 下面我们来说一下关系词可以省略的几种情况。
1关系代词在定语从句中作宾语时可以省略。当关系代词who, whom, which和that在定语从句中用作动词宾语或介词位于句末时的介词宾语时，可以省略。
如：
Is there anything (that) you wanted? (that作宾语)
Who is the man (that / who / whom) you were talking to? (that / who / whom作宾语)
2关系代词在定语从句中作表语时可以省略。
如：
China is not the country (that) it was. (that作表语)
3关系代词在定语从句中作宾补时可以省略。
如：
I’m not the madman (that) you thought me. (that作宾语补语)
4关系副词在定语从句中作状语时可以省略，有三种情况。
1、省略when的情况。用作时间状语的关系副词when通常不能省略，但用于day, year, time，moment等少数几个词后时可以省略，也可换成that。如：
That was the year (that) I first went abroad.
I’ll never forget the day (that) we met.
2、省略where的情况。用作地点状语的关系副词where通常不能省略，但用于place, somewhere, anywhere, everywhere, nowhere少数几个词后时可以省略, 也可换成that。如：
This is the place (where) they met yesterday.
Have you somewhere (that) I can lie down for an hour?
3、省略why的情况。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，均可省略。如：
That’s the reason (why/ for which/ that) he came.
Give me one reason (why) we should help you.
考研英语长难句中定语从句的关系词省略的情况只适用于限定性定语从句，非限定性定语从句不可以省略关系词。