1. for因为，由于……的缘故；既然；鉴于……

a) You'd better put on your sweater, for it's rather cold outside.

你最好把毛衣穿上，外边相当冷。

b) He felt no fear, for he was a brave man.

他不感到害怕，因为他是个勇敢的人。

c) Comfort him for he is sad.

去安慰他，因为他很悲伤。

比 because正式一些，少用于口语中，用于提出证据或说明前面刚作的陈述。

判断改错：

(错) For he is ill, he is absent today.

(对) He is absent today, for he is ill.

2. because

1）因为，由于

The boy was always late because it took so long to get dressed.

这个男孩因花太长时间穿衣服总是迟到。

I can't do it now because I am busy.

因为我正忙着，现在不能做此事。

2）用于引导一个名词从句，相当于 that或 the fact that…，这一事实，这一点原因，原因是

The reason I am here is because I want to ask for your help.

我之所以到这儿来是想寻求你的帮助。

3）用于省略句，表示暂不便说明详细理由

I think it would make her feel better because.

我想它会使她觉得好受些。原因嘛，现在不说。

评：虽然都表示因为，但是 for更正式哦！