副词
1. How long since is it?
那是在多久以前？
介词
1. I haven't heard from him since last year.
我自去年以来未曾收到过他的信。
连词
1. It's been ten years since they married.
他们结婚到现在已经十年了。
2. What have you been doing since I last saw you?
自我上次和你见面以后，你在做什么？
3. He must have taken the book since it isn't here.
他一定是把书拿走了，因为书已不在这里了。