个人总结:
其实主要还是在非限制性定语从句， 在定语从句中作主语， 与介词连接的时候 一般先行词不省略， 做补语， 状语 定语从句中做宾语，大部分情况都可以省略， 下方与介词连接的时候注意有两种， 一种与介词连在一起的不能省， 一种介词在后边与动词连接， 关系代词自己放句子最前边的情况下， 关系代词也可省略。
一、 英语定语从句中关系词的省略与不省略。
1. 关系代词
Which, who, whom, that 在定语从句中作宾语时可以省略。
.：Here is the man (who/ that/ whom) you’ve been looking for.
以下情况不能省略：
（1） 介词＋whom 与 介词 ＋ which 中， whom, which 不能省略。
.：Do you know any shops nearby in which I can buy flowers?
.: That’s the headmaster with whom my father was talking just now.
注意：当介词放在动词后时，which, whom 就可以省略。
.: Who is the man (whom) you talked with just now.
(2).在非限制性定语从句中，作宾语的关系代词也不能省略。
.: Mr. Green, whom you know, is the tallest in our school.
.: The elephant looks like a spear, as anybody can see.
(3).在 the same…as , such…as, as…….as , the same….that中， as, that, 即使作宾语，也不能省略。
.: I have bought the same bicycle as you have( 这里可以理解为省略bought) (指同类用as)
The same …….as : 与…..相同/一样/一致。
This is the same book that I read the day before yesterday. (that 指同一个)
（4）当由and, but, or 等连词连接两个或两个以上的定语从句修饰一个先行词时， 第一个关系代词可省，但第二、第三个不可省略。
.: This is the book (which) I read yesterday and which I find very interesting.
二、 当 that 在从句中作补语时可省略。
.: He is not the man (that) he used to be.
.: She is all (that) a teacher should be.
三．在there be 结构中出现定语从句，或 在定语从句中出现there be 时， 用作主语的关系代词也可省略。
.: There is a man downstairs ( who) wants to see you.
.: This is the best dictionary (that) there is in the library.
四、状语的省略。
（1）当先行词是reason， 而且定语从句在句中作原因状语时， 关系代词可用 why , that，也可以省略。
.: the reason (why/that) he failed,……..
(2) 当先行词是way， 且后边的定语从句再句中作方式状语时， 关系代词可用 in which/ that, 也可省略。
.: The way ( in which/that) these comrades look at problems is wrong.
.: That was the way (in which/that) she worked the problem out.
注意：当关系代词在定语从句中作主语时， 不省略。
.: I don’t know the way that/which leads to the top of the mountains.
(3) 当先行词时time时， 关系代词可用when/that/ 或省略
.: The second time (that) I saw him was in 1980.
.: I don’t know the exact time (when/that) the sports meet.
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