since的用法时态是怎样的呢？下面就让我们一起来了解一下吧：
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.
