1） because语势最强,用来说明人所不知的原因,回答why提出的问题,是句子中的重要部分,常放在句末,不能用其它三个替换.逗号可加可不加.
You can trust those products because the quality never changes.
I haven”t been to the cinema lately,because I am too busy.
I didn”t go,because I was afraid.
2）当原因是显而易见的或已为人们所知,就用as或 since.since稍正式一些.
Since /As the weather is so bad,we have to delay our journey.
3） 由because引导的从句如果放在句末,且前面有逗 号,则可以用for来代替.但如果不是说明直接原因,而是多种情况加以推断,就只能用for.
He is absent today,because / for he is ill.
He must be ill,for he is absent today.
4）当because引导的从句与否定的主句连用,而主句又位于句首时,之前不用逗号,因为主句动词否定的是其后的全部内容.为了突出because从句,because 前常加上副词,如：just,only,simply,chiefly等.在这种情况下,常常会形成否定的转移.如果是在口语中,在because前要有一个停顿（逗号）,才能表示出否定没有转到because从句中.如：
You should not despise a man just because he is poorly dressed.
She is not marrying Bill,because he is poor.
because引导的从句可以被just,only,simply等副词修饰,也可以用并列连词构成并列的 because从句,而since和as则不能.如：We pardoned him only because he was still young.for表示原因时,置于主句之后,表示非直接的随后附加说明的理由或推断理由.如：She didn’t go to school,for she was ill.