since的用法时态

since作为副词形式一般用于现在完成时或过去完成时，作为介词形式一般用于现在完成时或过去完成时。

用法：

1、since+过去一个时间点（比如具体的年、月、日期、钟点等）

例：I have been here since 1990.

2、since+一段时间+ago

例：I have been here since six months ago.

3、since+从句

例：Great changes have taken place since you left.

4、It is+一段时间+since从句

例：It is one years since I became a postgraduate student.