for 有介词和连词两种用法
介词如：what can I do for you.
I'm waiting for you.
I have stay here for years.
做连词时,其用法和because基本等同
如I beat him,for he made me irate.
但通常情况下,because作为连词意味因为比较普遍.
so作为连词时意思为“那么,因此,所以”,后面强调的是结果
It's raining hard,so I won't get to school on time.
也可以做副词,表示,“如此,如是”
如Time goes so quick.
I like to do so.