比较：because, since, as和for 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。
首先， "because" 的用法与"since" 及"as" 相同，可以写在句首，或者写在主句(Main Clause)这后。如： Since/AS/Because it was getting hot, he took off his coat。
=He took off his coat since/as/because it was getting hot。 其次，注意since, as和because置于句首时，要用逗号，置于Main Clause之后则不用。 最后，比较：because, since, as和for 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。
当原因是显而易见的或已为人们所知，就用as或 since。 I didn't go, because I was afraid。 Since /As the weather is so bad, we have to delay our journey。
2） 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。 摘自： 。
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