1. 关于 because

语气最强，表示直接原因，可用于回答 why 提出的问题、引导表语从句、用于强调句等，而其余三者均不行。如：

A：Why did she love him? 她为什么爱他?

B：Because he was very rich. 因为他很有钱。

She suffers from boredom because she has nothing to do. 她感到厌烦，因为她无事可做。

The room doesn’t get much light because the windows are so small. 因为窗子太小，房间里光线不足。

2. 关于 since, as, now that

(1) 三者所表示的原因都是人们已知的，即对已知事实提供理由，而不是表示直接原因。since 比 as, now that 语气稍强，且比 as, now that 略为正式，它们引导的从句通常放在主句之前，有时也放在主句之后。如：

As you weren’t there, I left a message. 由于你不在那儿，我留了个口信。

Since you ask, I will tell you why. 既然你问起来了，我就告诉你为什么。

Now that John’s arrived, we can begin. 既然约翰来了，我们就可以开始了。

放在主句后的例句:

The discussions were a farce since the minister had already made the decision. 讨论是一场闹剧，因为部长早已作出了决定。

You’re really in a mess now that you’ve been caught cheating. 你因为骗人被抓住了，这下子可真麻烦了。

(2) since 可用于省略句，而其他三者不行。如：

Since so, I have nothing to say. 既然如此，我无话可说。

3. 关于 for

是并列连词(其余三者为从属连词)，它有时可表示因果关系(通常要放在主句之后，且可与 because 换用)； 有时不表示因果关系，而是对前面分句内容的解释或推断(也要放在主句之后，但不能与because 换用)。比较：

The shops were crowded, for it was near Christmas. 店铺里挤满了人，因为圣诞节快到了。

She does not go out in the winter, for she feels the cold a great deal. 她冬天不出门，因为他特别怕冷。

It must have rained last night，for the ground is wet this morning. 昨晚一定下过雨，你看今天早上地面是湿的。(此句不能用 because 代 for)

4. 习惯表达

汉语习惯上说“因为……所以……”，但是在英语里却不能将 so 与because, since, as 等连用：

因为天气很好，所以我们去游泳了。

误：Because it was fine, so we went swimming.

正：Because it was fine, we went swimming.

正：It was fine, so we went swimming.

因为很迟了，所以我要回家了。

误：Since it is late, so I shall go home now.

正：Since it is late, I shall go home now.