1) 關(guān)于 because：語氣最強(qiáng),表示直接原因,可用于回答 why 提出的問題、引導(dǎo)表語從句、用于強(qiáng)調(diào)句等,而其余三者均不行：
“Why didn’t he come?” “Because he was ill.” “他為什么沒來?”“因?yàn)樗×?”
My stomach hurts because I have eaten too many apples.我肚子痛,因?yàn)樘O果吃得太多.
That’s because you can’t appreciate music.這是因?yàn)槟悴荒苄蕾p音樂.
It is because he is honest that I like him.是因?yàn)樗\實(shí)我才喜歡他.
(2) 關(guān)于 since 與 as：
a.兩者所表示的原因都是人們已知的,即對已知事實(shí)提供理由,而不是表示直接原因.since 比 as 語氣稍強(qiáng),且比 as 略為正式,它們引導(dǎo)的從句通常放在主句之前,有時也放在主句之后：
As you weren’t there,I left a message.由于你不在那兒,我留了個口信.
Since you are wrong,you should apologize.你既然錯了,就應(yīng)該道歉.
b.since 可用于省略句,而其他三者不行：
Since so,I have nothing to say.既然如此,我無話可說.
(3) 關(guān)于 for：是并列連詞(其余三者為從屬連詞),它有時可表示因果關(guān)系(通常要放在主句之后,且可與 because 換用)；有時不表示因果關(guān)系,而是對前面分句內(nèi)容的解釋或推斷(也要放在主句之后,但不能與because 換用).比較：
The ground is wet,for (=because) it rained last night.地面是濕的,因?yàn)樽蛲硐逻^雨.
It must have rained last night,for the ground is wet this morning.昨晚一定下過雨,你看今天早上地面是濕的.(此句不能用 because 代 for）