1. 作从属连词引导时间状语从句，表示“当”、“一边……一边”。
I pulled a muscle as I was lifting a heavy suitcase.
Parts are replaced as they grow old.
注意：as不能用来表示在某一时间，如 As I started work here, the pay was $4 an hour. (x) 而应说When I started work here, the pay was $4 an hour.
2. 作从属连词引导原因状语从句，表示“因为”，等同because, since。
As the gorilla is so big and powerful, it has no real enemies.
As 常用于句首，但是当表达某件事的某一特殊原因时，在as前面可以用especially或particularly，此时as 引导的原因状语从句放在主句后面，如 I was frightened when I went to bed, especially as my room was so far up.
3. 作从属连词引导方式状语从句，置于主语之后，表示“按照”、“根据”。
Type this again as I showed you a moment ago.
4. 作从属连词引导比较状语从句，表示“像……一样”。
He looked over his shoulder as Jack had done.
5. 作副词，用于形容词前，表示“被认为”。
They regarded manual work as degrading.
6. 作介词，表示“以……身份”、“作为”、“当作”。
He is greatly respected both as a person and as a politician.
7. 作介词，表示“像”。
The bread is as hard as a brick.
注意：通常在将一件事或人同另一件事或人进行比较时，名词词组前不用as而用like.
He swims like a fish.
Children, like animals, are noisy at meal times.