1. Non-Restrictive（非限制性） vs Restrictive attributive （限制性）： Differences
1.1. 1. How closely related to the modifier
1.1.1. R is more closely while NR is not so close and can be omitted from the sentence
1.1.1.1. i.e.:Li Ming, who is in the room, wants to ask you some questions. (NR can be omitted) This is the man who gave me the book. (Being omitted, the sentence won't stay complete)
1.1.2. We don't use that and why to start a NR
1.1.2.1. i.e. I have to tell them the reason, __________ I didn’t attend the meeting. for which instead of why
1.2. 2. Modify the whole sentence or just the antecedent(先行词)
1.2.1. R is only for the antecedent while NR can be used for the whole sentence
1.2.1.1. He passed the college entrance examination, which pleased his father a lot. (The whole thing：passed the exam pleased his father)
1.3. 3. Substitution 替代关系
1.3.1. in R : we can use who to replace whom , but not in NR ones.
1.3.1.1. This is the girl who/whom I met yesterday.
1.3.1.2. A young man had a new girl friend, whom he wanted to impress.
1.4. 4. Meaning 意义上的不同
1.4.1. 1.She has two brothers who are working in the city. 她有两个在这个城市工作的弟弟。 (She might have more brothers) 2.She has two brothers, who are working in the city. 她有两个弟弟, 他们在这个城市工作 ((She has only two brothers.) )
1.5. 5.省略 ； 关系代词在R中做宾语，可以省略在NR中不能
1.5.1. This the the girl (who )I met today.
2. Omission
2.1. Relative Pronoun
2.1.1. 1. as object in the clauses
2.1.1.1. Is there anything (which) you want? 想要什么东西吗?
2.1.2. 2.as predicative
2.1.2.1. China is not the country (that) it was. 中国已不是过去的中国了。(that作表语)
2.1.3. 3.object complement
2.1.3.1. I’m not the madman (that) you think me. 我并不是你所认为的那个疯子。(that作宾语补语)
2.2. Relative adverb
2.2.1. 1.When（after：key words: day, year, time ）
2.2.1.1. I’ll never forget the day (when) we first met(met for the first time). 我永远也忘不了我们初遇的那一天。
2.2.2. 2.Where（after：place, somewhere, anywhere, everywhere, nowhere）
2.2.2.1. This is the place (where) they met yesterday. 这就是他们昨天碰头的地方。
2.2.3. 3.Why (after: the reason)
2.2.3.1. That's why he came = That’s the reason (why, for which, that) he came 这就是是他来的原因