定語從句的引導詞包括關係代詞，如that, which, who, whom, whose, as等、關係副詞，如when, where, why等。其中，以that和which最為常見，而that和which在有的情況下可以互換，但在有的情況下是不能互換的，下面我們來總結一下它們的用法及辨析吧。
That：
1.先行詞既可是人，也可是物（相當於who, whom, which)
2.既可作主語，也可作賓語（做賓語時可省略）
eg. Where is the man that(whom) I saw this morning?
Which:
1.先行詞必須是物
2.既可作主語，也可作賓語（做賓語時可省略）
eg. This is the pen(which) I bought yesterday.
只用that的情況：
1.當先行詞是不定代詞（複合不定代詞） ，如：anything, everything, nothing, something, few, all, none, little, some, any, no, much等或由不定代詞any, all, some, no, little, few, much等修飾時。
eg. All that can be done has been done.
eg.There is nothing that I can do for you.
2. 當先行詞被序數詞修飾時。
eg.The first place that they visited in London was the Big Ben.
3. 當先行詞被形容詞最高級修飾時。
eg. This is the best film that I have seen.
4.當先行詞被the very, the only, the right, the next 等修飾時。
eg. This is the very dictionary that I want to buy.
5.當先行詞前有who, which等疑問代詞時。
eg. Who is the man that is standing there?
6.當先行詞由兩部分組成（由and連接）且這兩部分包括人和物時。
eg. Can you remember the scientist and his theory that we have learned?
只用which的情況：
1.在非限制性定語從句中。
eg. China, which was founded in 1949, is becoming more and more powerful.
2.在介詞+關係代詞的定語從句中。
eg. The school in which I once studied is very famous.
3.先行詞為that, those時。
eg. What’s that which is under the desk?
Note:
1.先行詞：定語從句中被修飾的名詞或代詞叫做先行詞，一般放在引導詞前面。
2.非限制性定語從句：定語從句有限制性和非限制性兩種。限制性定語從句是先行詞不可缺少的部分，去掉它主句意思往往不明確；非限制性定語從句是先行詞的附加說明，去掉了也不會影響主句的意思，它與主句之間通常用逗號分開，將非限定性定語從句放在句子中間，其前後都需要用逗號隔開。
練習：
1. You should hand in all _________you have.
2.Please send us all the information ________you have about the candidate for the
position.
3. What is the first American film _______you have seen?
4.The first place________ they visited in London was the Big Ben.
5. This is the best________ has been used against pollution.
6.This is the most exciting film _______I ever seen.
7. They talked about the persons and things ______ they remembered at school.
8. Which is the T-shirt _______ fits me most?
9. That is the very book _____he is looking for.
10.He is the only man _______ can do the work.
11. Tom was late for work again this morning，_______ made the boss angry.
12. Football, ________is a very interesting game, is played all over the world.
13. This is the place in _______ Lu Xun used to live.
14.The chair on ______ she sat is made of wood.
15.He lived in a big house, in front of _______ stood a big tall tree.
答案： 1. that2. that3. that4.that5.that 6. that 7. that 8. that 9. that 10. that 11. which 12. which 13. which 14. which 15. which
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