since的四种用法
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）.
例如：I have been here since 1989. 1989起,我一直在这儿.
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,变化可真大.
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.
