1.限制性定语从句中关系代词作宾语时可以省略，非限制性定语从句中关系代词不能省略。
2.在非限制性定语从句中，指物时只能用which：
She sang a new song, which we liked very much.
3.定语从句由介词+关系代词引导时，只能用which
Water takes the shape of the container in which it is placed.
1.关系代词在从句中作主语时，从句中的谓语动词用单数还是复数由先行词决定。
2.定语从句有时不直接紧跟着先行词，中间由一个定语、状语或谓语隔开，这种从句可叫作分隔式定语从句
The days are gone forever when China relied on foreign oil.中国依靠洋油的日子一去不复返了。
3.引导定语从句的关系副词可用介词+which来代替，where=in which, when=on which
4.注意介词与关系代词的位置，介词在关系代词前，只能用which或whom且不能省略。介词在句尾，关系代词可用which, that, whom， 口语中也可用who且可省略。
His sister married the man to who she was engaged.
His sister married the man whom/who/that she was engaged to.
他姐姐嫁给了和她订婚的男子。
5.关系代词和关系副词在从句中充当一个成分，注意在从句中避免句子成分重复出现
The woman that you saw her in the park is our English teacher. 你在公园里见到的那位妇女是我们的英语老师。
This is the classroom where we have our classes in. 这是我们上课的教室。
6.在定语从句中作介词的宾语的关系代词which或whom省略时，介词不能前置
He often tells us stories people laugh at.
7.当which, whom在定语从句中作含有介词的短语动词的宾语时，介词不能前置，必须和动词放在一起。
❌My eldest brother who works there is fifty years old.
当先行词是独一无二的人或事，或是专有名词，或是带有类指的名词，或是整个主句时，只能用非限制性定语从句。“大哥”只有一个，无须限制。因此，应用非限制性定语从句。
❌Mr. Yu, who we’ve learned a lot from, is an old worker.
在非限制性定语从句中，关系代词前的介词一般不能后置，介词和关系代词也不能省略。
The two thins about which Marx was not sure were the grammar and some of the idioms of English.
介词about是与形容词sure搭配而成。
1.当定语从句中缺主语或及物动词的宾语时（间或作表语），不用介词
The mountain village, which I visited 10 years ago, has changed a lot in the past two years.
2.当定语从句的结构是“主语+谓语动词”或“主语+谓语动词+宾语”时，需用介词+关系代词，使之构成定语从句中的状语
The man to whom I spoke just now is a director of a workshop.
介词的确定取决于定语从句中的动词。
3.当定语从句中缺时间、地点、原因状语时，可用介词+关系代词代替关系副词when, where, why。
at/in/on which代替where
This is the place where/in which he stayed when he was in Malta.
at/in/on which代替when
This is the day when/on which he did experiments.
for which代替why
This is the reason why/for which he is leaving so soon.
另外当the way作先行词时，可用in which
I don’t like the way/in which you laugh at her.
以上结构中关系副词+关系代词（常可省略），其介词的确定是根据先行词而定。
At last there was something about which to write home.
更多定语从句例句可参考这里。
It was in the hall ____ the meeting was held.
A. that B. where
解析
如果该题无介词in，则应选where构成定语从句。区别主要在于：
1、强调句式It is/was……that/who可去掉，不影响句意，而定语从句不能。
2、It is/was……后，若是名词及其词组可以往定语从句上考虑；若是副词、介词短语则是强调句式。
We heard the news ____ our team had won.
A. which B. that
解析
从引导词判断，定语从句中的引导词除连接从句外，还在句中充当一个成分。that指物时也可用which取代，作宾语可省略。而同位语从句中的引导词，只起连接主从句的作用，不充当任何成分，that不能省，也不能用which代替。
He’s such a good teacher ____ we all love and respect.
A. that B. as
解析
此例句为定语从句，句末加him则选A为结果状语从句。（结果）状语从句中的谓语动词后一般需接宾语。
