1. as引导的原因状语从句一般表示为人熟知的原因,且该原因从句一般放在主句之前.
如：As he is blind, we often help him.
2. for表示原因时,有两种用法：
1）for做介词,加名词/代词做其宾语,但并不能引导原因状语从句.
如：He was late for this reason.
2）for做连词,引导原因状语从句,但是它只表示补充说明/解释,其理由并不一定充分,而且从句必须置于主句之后.
如：He may be in, for the light in his room is on.
3. because只能引导原因状语从句,表示的最直接/明显的原因,可以用于回答why提问的问句,而且从句的位置不限.
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视为介词,后面不能加原因状语从句,只能加名词/代词/动名词等构成介宾短语,也表示最直接/明显的原因.
如：I was late for school because of my mother/mother's being late.