如果原因已为听者/读者所知，或者它不是句子里最重要的信息，就用 as 和 since 。as 从句和 since 从句常常放在句首。

       As it's raining again, we'll have to stay at home. 因为又下雨了，所以我们只好待在

       家里。

      Since he had not paid his bill, his electricity was cut out. 由于他没有缴电费，他被

       断了电。

as  从句和 since 从句都是比较正式的说法；在非正式文体中，同样的意思往往用 so 来表示。

      It's raining again, so we'll have to stay at home.  因为又下雨了，所以我们只好待在

      家里。

     （摘自《牛津英语用法指南》第四版377.1节）