because,for,since,as的区别

ecause语气强,表示客观必然原因：
He is absent,because he is ill.因为生病,所以他没来.
比较:He is absent,for he is busy.（“生病”是“缺席”的必然原因,“忙”不是必然原因.）
for 语气轻,表示非客观必然的原因,是主观可改变结果的原因,甚至是猜测可能的原因:
He must be ill,for he is absent.“缺席”不一定是“生病”,只是交流猜测.
for 不能放句首,它是并列连词.
since,as 都是不讲自明的原因,是已知的原因.
Since I am a boy,let me carry the case.
As you don‘t feel well,you had better stay at home.