since用法：
1）since+过去一个时间点（如具体的年、月、日期、钟点、1980，last month，half past six）。
2）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。
3）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。
4）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。