1) since +過去一個(gè)時(shí)間點(diǎn)　(如具體的年、月、日期、鐘點(diǎn)、1980, last month, half past six)。
     I have been here since 1989.
     2) since +一段時(shí)間+ ago
     I have been here since five months ago.
     3) since +從句　
     Great changes have taken place since you left.
     Considerable time has elapsed since we have been here.
     4) It is +一段時(shí)間+ since從句
     It is two years since I became a postgraduate student.