它们语气强弱依次为because,since为从属连词.for 为并列连词,连接并列句
because表直接原因,回答why的问题.通常放在主句之后,可单独使用
We stayed at home because it rained
since侧重主句从句的原因很显然或已为人所知
Since everyone is here ,let's start
as 鉴于主从并重,从句说原因,主句说结果
As it is late.I must go
for由于,表明附加理由,从句不放于句首
He must be ill,for he is absent today