since怎么知道后面加没加从句 例如我这题。

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）。I have been here since 1989.2.since+一段时间+agoI have been here since five months ago.3.since+从句Great changes have taken place since you left.Considerable time has elapsed since we have been here.4.It is+一段时间+since从句It is two years since I became a postgraduate student.