没有什么区别,完全是根据句意来的.都可以表示【自从】,只是since做连接词的时候,还可以表示原因,引导原因状语从句.
介词 prep.
1.(常用于现在完成时或过去完成时)自...以来,从...至今
I haven't heard from him since last year.
我自去年以来未曾收到过他的信.
连接词 conj.
1.(常用于现在完成时或过去完成时)自...以来,从...至今
It's been ten years since they married.
他们结婚到现在已经十年了.
She has been living a hard life since her husband died.
她自从丈夫死后,一直过着艰苦的生活.
2.引导原因状语从句,相当于now that,意思是“因为……,既然……,鉴于……”.
He must have taken the book since it isn't here.
他一定是把书拿走了,因为书已不在这里了.
Since we are young,we shouldn’t be too afraid of making mistakes.
因为我们还年轻,所以不应该太害怕犯错.