1) since +過去壹個時間點 (如具體的年、月、日期、鐘點、1980, last month, half past six)。
I have been here since 1989.
2) since +壹段時間+ 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 +壹段時間+ since從句
It is two years since I became a postgraduate student.
since的用法主要有兩點：壹是表示“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並非如此，那麽正確的含義是什麽呢？
下面談談since作連詞引導時間狀語從句的用法。
壹、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“從動作開始的那壹時刻起”。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自從他們離開這裏，我們就壹直很想念他們。
這是最常見的壹種用法，此處不再多述。
二、若since引導的狀語從句的謂語動詞是持續性動詞或表示狀態的動詞的過去時時，則從句表示的時間是“從那持續動作或狀態結束時算起”。如：
I haven’t heard any noise since I slept. Sleep 為持續性動詞，sleep的動作結束時，即“醒來”時，這句應譯為“我醒後還未聽到任何聲音”。
John is now with his parents in New York, it is already three years since he was a teacher約翰現在和父母壹起住在紐約，他不當教師已經三年了。
本文開始出現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?妳離開（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性動詞的現在完成時，則表示動作和狀態延續到現在（說話時刻），其表示的時間的起點應從動作發生之時算起。如：
I haven’t heard from him since he has lived there。
這裏has lived表示動作的持續性，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，我就壹直沒收到他的來信”。
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引導的從句持續動詞的壹般過去時和現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，而後者的形式與內容是壹致的。
至於It is +壹段時間+since…的句型與It has been +壹段時間+since…意思相同。前者是規範說法，後者是口語說法。
應註意的是，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. 妳有兩個月沒來看我了。