As能引導幾種從句：
1. As可以引導定語從句。有時可譯為“正如”。此時，常常是as從句在前，主句在后。
例如：As we can see, he is a Chinese.
2. As 還可引導原因狀語從句，“因為，由于”
例如：As he is so lazy, he never cooks by himself.
3. As還可引導時間狀語從句，“當，隨著”
例如：As the weather is colder and colder, the leaves of the tree is less and less.
4. As還可引導方式狀語從句，“按照”
例如：You must do the work as I have told you.
5. As還可引導讓步狀語從句。“盡管”。但是注意，as引導方式狀語從句時從句必須倒裝。而且這種倒裝和一般的倒裝不一樣。as從句的倒裝一般是把從句的表語或作狀語的副詞提到as之前，（如果表語是一個帶有不定冠詞a或an的單數名詞，倒裝后a或an必須省略)。
例如：Though he is a little boy, he knows a lot.如果變成由AS引導，就必須變成：
Little boy as he is, he knows a lot.
Though he works hard, he never succeed.
可變為 Hard as he works, he never succeed.