1. because的用法
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。如果原因是句子里最重要的部分,通常将because从句放在句末。because从句也可以独自成句。since和as则不能这样用。
如:
A:Why didn’t she come? 她为什么没来?
B:Because she was very busy. 因为她很忙。
It is because he is clever that I like him. 是因为他聪明我才喜欢他。
Why am l leaving? I’ll leaving because I’m fed up!
我为什么要走呢?我要走,是因为我待够了!
Because I was ill for six months, I lost my job.
因为我病了六个月,所以把工作丢了。
2. since与as
两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 语气稍强,且比 as 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’ve been here awhile, you might as well stay. 既然你已经来了一段时间,就不妨继续待下去。
另外,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. for的用法
for是并列连词,而其余三者为从属连词。for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 ground is wet,for (=because) it rained last night. 地面是湿的,因为昨晚下过雨。
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)
We rarely stay in hotel, for we can’t afford it. 我们很少住旅馆,因为我们住不起。
When I saw her in the river I was frightened. For at that point the currents were dangerous.
for引导的从句不能位于not,but或任何连词之后:
He stole, not because he wanted the money but because he liked stealing.
他偷东西,并不是因为他想要钱,而是他有这种毛病。(这里不能用for。)