It is +一段时间+ since从句
It is two years since I became a postgraduate student.
It is + 一段时间 since + 瞬间动词
表从...已经有多少时间了
如: It is ten days since I saw him. 从上次我看到他已经10天了
但要注意的是如果since后面是持续动词则表示动作结束的时间
如: It is two months since he smoked. 他不抽烟已经2个月了.
It was +一段时间 before sb. did
表示过去某人过了...做
如:It was two days before he came back. 他过了两天回来了