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【关系代词和关系副词在用法上的区别】
1) This is the city which I live. *
2) This is the city which I live in.
3) This is the city where I live.
4) This is the life which I want to live.
- 以上例句中的 which 和 where 均在各自从句中代替先⾏词 city。
1) 之所以错误是因为从语法角度来看 which 是代词且作从句 which I live 的宾语，然⽽ which 所代替的先⾏词city 不可以作动词 live 的宾语（相当于 I live the city. *) 。
2) 之所以正确是因为在从句中which 作的是介词 in 的宾语，which 和 in ⼀起构成介词短语修饰 live（相当于 I live in the city. ) 。
3) 之所以正确是因为 where 在定语从句中作副词，可以直接修饰动词 live（相当于 I live here. ) 。
4) 之所以正确是因为 which 的先⾏词 life 可以作从句中 live 的宾语（相当于 I want to live this life.）。
【限制性定语从句和非限制性定语从句】
定语从句可分为限制性定语从句和非限制性定语从句。形式上，限制性定语从句与主句间不用标点分隔，而非限制性定语从句和主句间要由逗号或破折号隔开；意思上，限制性定语从句对先行词起限定作用，如果去掉会导致句子核心意思不完整，而非限制性定语从句对先行词起附加解释作用，即便去掉也不影响主句的核心意思。如：
1) Students who are struggling with vocabulary will find the test difficult.
2) Students, who are often struggling with vocabulary, will find the test difficult.
(1) 中定语从句为限制性定语从句，对先⾏词 students 起限定作用，句⼦意思⼤概相当于“只有那些词汇学得不好的学⽣才会认为考试难”；
⽽ （2） 中的定语从句为非限制性定语从句，对先⾏词 students 仅起附加解释作用，句⼦意思相当于“学⽣会认为考试难，⽽且学⽣经常词汇学得不好。”
【不定式】
在某些情况下不定式的 to 会省略掉。如：
I heard you sing.
I saw you leave the house.
He help me do my homework.
You had better finish your homework now.
下划线所标记的部分为省略了 to 的不定式或不定式词组。
【省略】
1. 若并列结构中存在重复的内容则只保留一个。如：
I like swimming but Tom doesn’t like swimming.
Your words made me happy but they made Tom angry.
应改为：
I like swimming but Tom doesn’t.
Your words made me happy but Tom angry.
2. 若状语从句同时满足 1）主语是 it 或主语与主句的主语一致，2）谓语包含 be，则状语从句的主语可以和 be 一起省略，从而将状语从句改写为分词结构。如：
You won’t feel it while you are asleep.
Bears normally will not attack you unless they are provoked.
You can ask him in person, if it is necessary.
可改为：
You won’t feel it while asleep.
Bears normally will not attack you unless provoked.
You can ask him in person, if necessary.
与之类似，若定语从句的关系代词在从句中作主语且定语从句谓语包含 be，则定语从句的关系
代词经常可以和 be 一起省略，从而将定语从句改写为定语、状语或同位语。如：
The boy who is sleeping over there is Tom.
Tom, who is big and fat, is not very good at table tennis.
Hans, who is raised in Germany, speaks both German and English fluently.
Mr. Smith, who is my math teacher, is tall.
可改为：
The boy sleeping over there is Tom.
Tom, big and fat, is not very good at table tennis.
Hans, raised in Germany, speaks both German and English fluently.
Mr. Smith, my math teacher, is tall.
3. 比较状语从句中的动词经常可以省略。如：
Tom is taller than Jerry is.
I run faster than he does.
I have done more work than you have.
可改为：
Tom is taller than Jerry.
I run faster than he.
I have done more work than you.