除了下面A2，A3中所示各种类型外，这两种从句均可由as或because来引导。但是用as引导原因从句较为稳妥;用because引导结果/原因从句较为稳妥(参见B)。
A 原因从句
1 由as/because/since 引导的原因从句：
We camped there as/because/since it was too dark to go on.
我们在那里露宿是因为天太黑，不能再继续往前走了。
As/Because/Since it was too dark to go on，we camped there.
因为天太黑不能再继续往前走，我们就在那儿露宿了。
2 in view of the fact that可用as/since/seeing that来表示，但不能用because：
As/Since/Seeing that you are here，you may as well give me a hand.
既然你在这儿，你就帮我个忙吧。
As/Since/Seeing that Tom knows French，hed better do the talking.
3 在as/since/seeing that意指以前共知的'或共知的陈述时，可用if来代替：
As/Since/Seeing that/If you dont like Bill，why did you invite him?
既然/如果你不喜欢比尔，你为什么邀请了他?
注意：if so的用法：
I hope Bill wont come.
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