because语气最强,since其次,as最弱
区别
a because表示直接理由.强调别人未知的事实,而since,as引导的原因状语从句一般表示已知的事实,强调引起的后果.
例如：
He had to to a part-time job after work because he needed money for his marriage.他下班后还得兼职因为他结婚需要钱.
Since you know you are wrong,you had better admit it.既然你知道你错了,你最好赶快承认.
As he couldn't express himself freely,he had to make some gestures.由于他不能表达自如,只得用手势.
尤其在讯问尚未确定的原因时,必须用because.例如:
Did he buy many books becasuse he was fond of reading?他买许多书是因为他喜欢看书吗?
b.because引导的从句可以带修饰语just,only,也可以用于强调据或not.but并列结构,但since,as不可以.
例如:
The girl learned to play the piano not because she liked to play the piano but because her parents forced her to.女孩子弹钢琴不是因为她喜欢弹琴,而是因为她父母强制她学.
It was because he lost it that he found it more valuable than ever.正因为他失去了它,他才比以往任何时候感到它的宝贵.