as 表原因后可跟名词短语或句子,但它不能用于句首.
because表原因后只可跟句子,它可用于句首或句子中间.
because of 后只可跟名词短语来表原因,可用于句首或句中间.
for只可跟名词短语,也只可用于句子中间.
例：
I so happy FOR your help.
I so happy BECAUSE OF your help.
I so happy AS your help.
I so happy,because you gave me a hand.