since与becouse的区别,以及具体用法

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.
1.since（既然）语气较弱,强调已知的事实.
例：Since you can't answer the question,I'll ask someone else.
既然你回答不出这个问题,我就问别人了.
2.for（因为）的用法：
for是并列连词,表示推断的原因,或对前一分句补充说明理由.
例：It must be morning for the birds are singing.一定是清晨了,因为鸟儿在叫.
3.because （因为）的用法：
because通常表示直接的原因.because引导的从句表示直接而明确的原因和理由,表示按因果关系的推断,语气很强,用来回答why的问题,一般位于主句之后.
例：We couldn't go out because it was too cold.因为天气太冷,我们不能外出.