since的时态搭配

　　用法

　　1since+过去一个时间点（如具体的年、月、日期、钟点、1980，last month，half past six）。

　　since+一段时间+ago.

　　例如：I have been here since five months ago. 我在这儿，已经有五个月了。

　　since+从句。

　　例如：Great changes have taken place since you left. 你走后，变化可大了。

　　Great changes have taken place since we were here.我们走后，变化可大了。

　　It is+一段时间+since从句。

　　例如：It is two years since I became a postgraduate student. 我考上研究生有两年了。