As,For,Because,Because of

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.

都可以表示因为。具体如下？
As可以=Because如：
As the weather is good, i would like to go shopping.
because the weather is good, i would like to go shopping.
for 和because of只能和短语连接使用，如:
I would lie to go shopping because of the good weather.
I would lie to go shopping for the good weather.

as,明显的理由 As he looks pale, he must be ill.
for,推测的理由(用在句中) The day breaks out, for the birds are singing.
because,一般笼统的、直接的原因，后面加从句
because of,同上，后面加名词／动名词

从语法上看：
as和because意思差不多,但是语气比because弱，一般表示明显的原因。
for一般只用于句尾，用逗号与主句隔开。
because是最普遍的，表示原因的大都可以用它，后面接that从句。
because of 后面接名词或者动名词。