关系词有时可以省略，但这只限于限制性定语从句中，在非限制性定语从句中，关系词则不可省略。
一、关系代词作宾语时的省略
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: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(which) you wanted?
想要什么东西吗?
Who is the man (that/who /whom) you were talking to?
刚才和你讲话的人是谁?
二、关系代词作表语时的省略
当关系代词that在定语从句中用作表语时，可以省略。
如:
China is not the country (that) it was.
中国已不是过去的中国了。(that作表语)
三、关系代词作宾补时的省略
当关系代词that在定语从句中用作宾语补足时，可以省略。
如:
I'm not the madman (that) you thought me.
我并不是你所认为的那个疯子。(that作宾语补
四、关系副词when的省略
用作时间状语的关系副词when通常不能省略，但有一种特殊情况，即用于day, year, time等少数几个词后时可以省略(也可换成that)也可换成。
如:
That was the year (that)l first went abroad.
就是那一年我第一次出国了。
!'ll never forget the day (that)we met.
我永远也忘不了我们见面的那一天。
五、关系副词where的省略
用作地点状语的关系副词where通常不能省略，但有一种特殊情况，即用于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.
这就是他们昨天碰头的地方。Have you somewhere (that)|can lie down for an hour? 你有没有一个什么地方可以让我躺一个小时?
六、关系副词why的省略
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.
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。
The reason women are drinking is just because they like it.
女人喝酒只是因为她们喜欢喝。
The reason our schools cannot afford better teachers is because people here are poor
我们的学校请不起好老师的原因是因为这里的人穷。
The reason many people fail on diets is that they jump in at the deep end, making a complete change to their eating habits.
很多人节食失败的原因是他们对节食一窍不通，一下子完全改变了饮食习惯。
The reason he didn't have a family was that he didn't want to be tied down.
他没有成家的理由是不想被捆住手脚。
七、关系代词后出现 there be 句型时
在关系代词后出现 there be 句型时，关系代词也经常省略
She taught me the difference [that] there is between what is right and what is wrong.
她教了我好坏的区别。
The number of mistakes [that] there are in this homework is simply astounding.
在这份课外作业中错误多得惊人。
I have seen all [that] there is left to see in the city.
在市内未看完的东西我全部看了。
八、有插入句I know，we think 等时
出现插入句I know，we think 等时，关系代词也经常省略
She is just the type [that] I always knew would attract him.
她确是我知道的能吸引他的那种典型的女性。
Many of those qualities [that] we think are typical of Americans in general were the result of this frontier life.
可认为是美国人一般的特征的许多那样的气质，是从边疆拓荒生活中产生出来的。
Here are a few beverages (that) you might think help your diet, but don't.
以下是一些你可能认为有助于饮食的饮料，但事实并非如此。
That was the post (that) I said was wrong.
那是我认为错误的帖子。
He is the man (who) people believe will be the next Prime Minister.
人们相信他将成为下一任首相。
另外要注意的是，如果非要不省略这里的关系代词，不能把关系代词放在插入语之后，必须放在之前。
九、在英语的定语从句中一般是不能省略充当主语的关系代词（who，that, which）的，但是，在在口语里，强调句型也经常省略作主语的关系代词that以及存在句型的定语从句中充当主语的关系代词生可以省略的。如：
There is a gentleman wants to see you.
有一位先生要见您。
There is a table stands in the corner.
墙角放着一张桌子。
There’s a man at the door wants to speak to you.
门口有个人想跟您谈话。
There's a lady at the gate of the school wants to see you.
学校门口有一位女士要见你。
Here's a lady called about a cat.
有位太太为着猫的事来访。
Here’s a little book will tell you how to raise roses.
这是一本关于玫瑰花栽培的小册子。
There were a lot of girl students of our class took part in the sports meet.
我们班的一些女学生参加了运动会。
It’s Simon did it.
这是西蒙干的。
It's not everybody knows the right way to learn foreign languages.
并不是人人都知道学外语的正确方法。
It was I bought these flowers for Miss Li
为李小姐买了这些花的人是我呀。
It’s not every boy gets the chance to go to the university.
这并非是每个男孩都有机会进入大学。
It isn’t everybody can do that.
并不是每个人都能干这件事。