11.13 since的四種用法

　 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.