（二）关系代词替代情况不同
关系代词 whom 在限制性定语从句中作宾语时可用 who 代替 whom ，但 whom 在非限制性定语从句中作宾语时不可用 who 来代替。
例 1. This is the girl whom I met in the street.
这是我在街上遇到的那个女孩。
析：先行词 the girl 在限制性定语从句中作宾语，可用 who 代替 whom .
例 2. A young man had a new girl friend， whom he wanted to impress.
一个年轻的小伙子新交了一个女朋友，他想给她留下深刻的印象。
析：先行词 a new girl friend 在非限制性定语从句中作宾语，不可用 who 代替 whom .
在限制性定语从句中，先行词指人时可用 that 代替 who/ whom ，但在非限制性定语从句中先行词指人时，不可用 that 代替 who/whom .
例：她有一个姐姐，她是教师。
误： She has a sister， that is a teacher.
正： She has a sister， who is teacher.
（三）关系代词省略情况不同
关系代词在限制性定语从句中作宾语时可以省去，非限制性定语从句的所有关系词均不可省。
例 1. This is the book （which/that） he lost yesterday.
这就是他昨天丢的那本书。
析：先行词 the book 在限制性定语从句中作宾语，关系代词 which 或 that 可以省略。
例 2. The book， which he lost yesterday， has been found.
他昨天丢了这本书，但现在已找到了。
析：先行词 the book 在非限制性定语从句中作宾语，关系代词 which 不可省。
［实战演练］
1. She heard a terrible noise， _________ brought her heart into her mouth.
A. it B. which C. this D. that
2. Have you ever been to Xi'an， _________ I left ten years ago.
A. which B. whose C. that D. /
3. The weather turned out to be very good， _________ was more than we could expect.
A. what B. which C. that D. it
4. His English， ________ used to be very poor is now excellent.
A. which B. that C. it D . whom
5. Some of the roads were flooded， __________ makes o
ur journey more difficult.
A. that B. it C. which D. who
6. I'll find a nice girl， _________ I want to marry.
A. who B. whom C. that D. which
7. I'll come at ten， ________ I'll be free.
A. which B. that C. when D. what
8. She has two brothers， _________ are teachers.
A. who B. that C. whom D. /
9. Which answer is NOT true？
This is the factory ________ I visited yesterday.
A. that B. which C. / D. where
10. I still remember the days _________ I spent in Beijing.
A. when B. what C. that D. during
Key：
1. B 2. A 3. B 4. A 5. C
6. B 7. C 8. A 9. D 10. C
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