［動(dòng)詞的時(shí)態(tài)］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)。例如：

　　I have been here since 1989. 1989起，我一直在這兒。

　　2) since +一段時(shí)間+ ago.例如：

　　I have been here since five months ago. 我在這兒，已經(jīng)有五個(gè)月了。

　　3) since +從句。例如：

　　Great changes have taken place since you left. 你走后，變化可大了。

　　Great changes have taken place since we were here. 我們走后，變化可大了。

　　4) It is +一段時(shí)間+ since從句。例如：

　　It is two years since I became a postgraduate student. 我考上研究生有兩年了。