1 because(因为）：表示直接的原因或理由,着重点在从句,用于回答why,语气最强.
eg.
He didn't go to school because he was ill.
Why didn't he go to school? Because he was ill.
2 since(既然）：一般表示对方已知的,无须加以说明的即成事实的理由,全句中心在主句,语气比
because弱.
Since you are ill, I'll go alone.
3 as(由于）：表示十分明显的原因,一般说明因果关系,着重点在主句,语气较弱.
As it is snowing, we shall not go to the park.
4 for(因为,其理由是）：是个并列连词,只能放在另一个并列句后面,表示推理或解释,或用作附加说明,表示新的情况,而不是指理由或原因,语气最弱,一般不放在句首.
The day breaks, for the birds are singing.
* for表示因果关系时,可以同because交替使用,但for前须用逗号,而because则不必.
You couldn't have seen him because he wasn't there.
= You couldn't have seen him, for he wasn't there.