1. as引導(dǎo)的原因狀語從句一般表示為人熟知的原因,且該原因從句一般放在主句之前.
如：As he is blind, we often help him.
2. for表示原因時,有兩種用法：
1）for做介詞,加名詞/代詞做其賓語,但并不能引導(dǎo)原因狀語從句.
如：He was late for this reason.
2）for做連詞,引導(dǎo)原因狀語從句,但是它只表示補充說明/解釋,其理由并不一定充分,而且從句必須置于主句之后.
如：He may be in, for the light in his room is on.
3. because只能引導(dǎo)原因狀語從句,表示的最直接/明顯的原因,可以用于回答why提問的問句,而且從句的位置不限.
如：I got up late because I had stayed up last night.
= Because I had stayed up late, I got up late.
4. because of視為介詞,后面不能加原因狀語從句,只能加名詞/代詞/動名詞等構(gòu)成介賓短語,也表示最直接/明顯的原因.
如：I was late for school because of my mother/mother's being late.