(1) as 引导的定语从句可置于句首，而which不可。
As we all know, he never smokes.
(2) as 代表前面的整个主句并在从句中作主语时，从句中的谓语必须是系动词;若为行为动词，则从句中的关系代词只能用which。
(3)非限定性定语从句中出现expect, think, suppose 等表示猜测、想象、预料等时。
She succeeded in her doing the research work , as we expected.
(4)As 的用法 the same… as; such…as 中的as 是一种固定结构, 和……一样……。
I should like to use the same tool as is used here.
We should have such a dictionary as he is using.
1. I still remember the night _______I first came to the house.
2. I'll never forget the day________ we met each other last week.
3. Mr Black is going to Beijing in October, _______is the best season there.
4. I will never forget the days _______I spent with your family.
5. I'll never forget the last day______ we spent together.
6. This is the school ______I used to study.
7. Do you still remember the place______ we visited last week?
8. Do you still remember the place_______ we visited the painting exhibition?
9. Have you ever been to Hangzhou,_____is famous for the West Lake?
10. Have you ever bee to Hangzhou, ______lies the West Lake?
11. Tom will go to Shanghai,______live his two brothers.
12. I live in Beijing,____is the capital of China.
13. There was a time ______there were slaves in the USA.
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