since用法：
1）since+過(guò)去一個(gè)時(shí)間點(diǎn)（如具體的年、月、日期、鐘點(diǎn)、1980，last month，half past six）。
2）since+一段時(shí)間+ago.
例如：I have been here since five months ago. 我在這兒，已經(jīng)有五個(gè)月了。
3）since+從句。
例如：Great changes have taken place since you left. 你走后，變化可大了。
Great changes have taken place since we were here.我們走后，變化可大了。
4）It is+一段時(shí)間+since從句。
例如：It is two years since I became a postgraduate student. 我考上研究生有兩年了。