since的用法
1) since +過去一個時間點　(如具體的年、月、日期、鐘點、1980,last month,half past six).
I have been here since 1989.
2) since +一段時間+ 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 +一段時間+ since從句
It is two years since I became a postgraduate student.
since的用法主要有兩點：一是表示“自從……以來”,二是表示“既然”之意.但如果只了解詞的含義,而不了解其用法,在使用時就會造成錯誤.例如：I have not heard from him since I lived in Shanghai.
不少人會望文生義,將該句譯為：“自從我住在上海以來,就再也沒有收到他的信了.”而實際上這個句子的譯文并非如此,那么正確的含義是什么呢?
下面談談since作連詞引導時間狀語從句的用法.
一、若since引導的狀語從句的謂語動詞是終止性的過去時,則從句表示的時間是“從動作開始的那一時刻起”.如：
He has studied very hard since he came to our school.自從他來到我們學校,他學習就非常努力.
We have been missing them since they left here自從他們離開這里,我們就一直很想念他們.
這是最常見的一種用法,此處不再多述.
二、若since引導的狀語從句的謂語動詞是持續(xù)性動詞或表示狀態(tài)的動詞的過去時時,則從句表示的時間是“從那持續(xù)動作或狀態(tài)結束時算起”.如：
I haven’t heard any noise since I slept.Sleep 為持續(xù)性動詞,sleep的動作結束時,即“醒來”時,這句應譯為“我醒后還未聽到任何聲音”.
John is now with his parents in New York,it is already three years since he was a teacher約翰現(xiàn)在和父母一起住在紐約,他不當教師已經三年了.
本文開始出現(xiàn)的那個句子就可以譯為“自從我離開上海,就再也沒有收到他的信.”這種譯法常被人們誤用,我們不妨再多舉幾個例子看看：
How long is it since you lived in Shanghai?你離開（沒住在）上海多久了?
It is half a month since he was a League member.他退團（不當團員）半個月了.
It’s been quite some time since I was last in London我離開倫敦已很長時間了.
比較：He has written to me frequently since I was ill.自從我病愈以來,他屢次給我寫信.（從句謂語為狀態(tài)動詞）
He has written to me frequently since I fell ill.自從我生病以來,他就屢次給我寫信.（從句謂語為終止性動詞）
三、若since引導的狀語從句的謂語為持續(xù)性動詞的現(xiàn)在完成時,則表示動作和狀態(tài)延續(xù)到現(xiàn)在（說話時刻）,其表示的時間的起點應從動作發(fā)生之時算起.如：
I haven’t heard from him since he has lived there.
這里has lived表示動作的持續(xù)性,時間的起點應從：“開始居住”時算起.因此此句可理解為“自從他（開始）住在那兒起,我就一直沒收到他的來信”.
Since we have owned a car,we have gone camping every year.自從我們自己有了汽車,我們每年都去野營.
試比較：1.Since I’ve been at this school,we have had three headmasters.自從我到這所學校,我們已換了三個校長了.
Since I was at this school,they have had three headmasters.自從我離開這所學校,他們已換了三個校長了.
2.He has never been to see me since I have been ill.自從我生病以來,他從未來看過我.
He has never been to see me since I was ill.自從我病愈以來,他從未來看過我.
由此可見,since引導的從句持續(xù)動詞的一般過去時和現(xiàn)在完成時所表示的意思恰好相反.換言之,前者以肯定的語法形式反映著否定的邏輯內容,而后者的形式與內容是一致的.
至于It is +一段時間+since…的句型與It has been +一段時間+since…意思相同.前者是規(guī)范說法,后者是口語說法.
應注意的是,在這種結構中,since引導的從句絕對不能用否定式的謂語動詞來表示否定意義,下面兩句都錯了：
It is a long time since he didn’t study English.
It’s two months since you didn’t come to see me.
正確的說法為：
It’s a long time since he studied English 他好久沒學英語了.
You haven’t come to see me for two months.你有兩個月沒來看我了.