此类连词主要有because, as, since, now(that)，等。because引导的从句表示产生某种结果的必然的因果关系，语气较重，可回答why问句；since语气较轻，常位于句首；as则语气最轻。
We couldn’t cross the river because the water had risen. 水已经上涨了，所以我们没能过河。
Since everyone is here, let’s begin. 既然大家都来了，我们就开始吧。
I must stop writing now, as I have rather a lot of work to do.我必须停笔了，因为我还有许多工作要做。
Now that you’ve got a chance, you might as well make full use of it.
The Italian boy was regarded as a hero because he gave his life for the country.
Why use wood when you can use plastic? 既然能用塑料, 为什么还要用木料?