1.現(xiàn)在完成時
現(xiàn)在完成時用來表示之前已發(fā)生或完成的動作或狀 態(tài)，其結果的確和現(xiàn)在有聯(lián)系。動作或狀態(tài)發(fā)生在過去 但它的影響現(xiàn)在還存在；也可表示持續(xù)到現(xiàn)在的動作或 狀態(tài)。其構成： have (has) +過去分詞。
2.比較過去時與現(xiàn)在完成時
1）過去時表示過去某時發(fā)生的動作或單純敘述過去的事情，強調(diào)動作；現(xiàn)在完成時為過去發(fā)生的，強調(diào)過去的事情對現(xiàn)在的影響，強調(diào)的是影響。
2）過去時常與具體的時間狀語連用，而現(xiàn)在完成時通常與模糊的時間狀語連用，或無時間狀語。
一般過去時的時間狀語:
yesterday, last week，…ago, in1980, in October, just now, 具體的時間狀語
共同的時間狀語:
this morning, tonight,this April, now, once，before, already, recently，lately
現(xiàn)在完成時的時間狀語
for, since, so far, ever, never, just, yet, till / until, up to now, in past years, always,
不確定的時間狀語
3）現(xiàn)在完成時可表示持續(xù)到現(xiàn)在的動作或狀態(tài)，動詞一般是延續(xù)性的，如live, teach, learn, work, study, know.
過去時常用的非持續(xù)性動詞有come, go, leave, start, die, finish, become, get married等。
舉例：
I saw this film yesterday.
（強調(diào)看的動作發(fā)生過了。）
I have seen this film.
（強調(diào)對現(xiàn)在的影響，電影的內(nèi)容已經(jīng)知道了。）
Why did you get up so early?
（強調(diào)起床的動作已發(fā)生過了。）
Who hasn't handed in his paper?
（強調(diào)有卷子，可能為不公平競爭。）
She has returned from Paris.
她已從巴黎回來了。
She returned yesterday.
她是昨天回來了。
He has been in the League for three years.
(在團內(nèi)的狀態(tài)可延續(xù))
He has been a League member for three years.
(是團員的狀態(tài)可持續(xù))
He joined the League three years ago.
( 三年前入團，joined為短暫行為。)
I have finished my homework now.
---Will somebody go and get Dr. White?
---He's already been sent for.
句子中如有過去時的時間副詞（如 yesterday, last, week, in 1960）時，不能使用現(xiàn)在完成時，要用過去時。
（錯）Tom has written a letter to his parents last night.
（對）Tom wrote a letter to his parents last night.
3. 比較since和for
Since 用來說明動作起始時間，for用來說明動作延續(xù)時間長度。
I have lived here for more than twenty years.
I have lived here since I was born..
My aunt has worked in a clinic since 1949.
Some new oilfields have been opened up since 1976.
I have known Xiao Li since she was a little girl.
My brother has been in the Youth League for two years.
I have not heard from my uncle for a long time.
注意：并非有for 作為時間狀語的句子都用現(xiàn)在完成時。
I worked here for more than twenty years.
(我現(xiàn)在已不在這里工作。)
I have worked here for many years.
(現(xiàn)在我仍在這里工作。)
小竅門： 當現(xiàn)在完成時+一段時間，這一結構中，我們用下面的公式轉化，很容易就能排除非延續(xù)動詞在完成時中的誤使。
1) (對) Tom has studied Russian for three years.
= Tom began to study Russian three years ago, and is still studying it now.
2) (錯) Harry has got married for six years.
= Harry began to get married six years ago, and is still getting married now.
顯然，第二句不對，它應改為 Harry got married six years ago. 或 Harry has been married for six years.
4. since的四種用法
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
5.延續(xù)動詞與瞬間動詞
1) 用于完成時的區(qū)別
延續(xù)動詞表示經(jīng)驗、經(jīng)歷； 瞬間動詞表示行為的結 果，不能與表示段的時間狀語連用。
He has completed the work. 他已完成了那項工作。 (表結果)
I've known him since then. 我從那時起就認識他了。(表經(jīng)歷)
2) 用于till / until從句的差異
延續(xù)動詞用于肯定句，表示"做……直到……" 瞬間動詞用于否定句，表示"到……，才……"
He didn't come back until ten o'clock.
他到10 點才回來。
He slept until ten o'clock.
他一直睡到10點。
典型例題
1. You don't need to describe her. I ___ her several times.
A. had met B. have met C. met D. meet
答案B. 首先本題后句強調(diào)對現(xiàn)在的影響，我知道她的模樣，你不用描述。再次，several times告知為反復發(fā)生的動作，因此用現(xiàn)在完成時。
2.---I'm sorry to keep you waiting.
---Oh, not at all. I ___ here only a few minutes.
A. have been B. had been C. was D. will be
答案A. 等待的動作由過去開始，持續(xù)到現(xiàn)在，應用現(xiàn)在完成時
關于since;
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(jīng)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(nèi)容，而后者的形式與內(nèi)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。
解析：6B 詞義辨析語句連貫盡（as long as）作者所能記憶的。Remember是持續(xù)性動詞，與表示持續(xù)意義的as long as（象……一樣長；只要……）連用，引導條件狀語從句。as soon as，when，since三項均表示短暫意義，通常與短暫性動詞連用。 （2）現(xiàn)在完成時可以和帶有since或for等表示“一段時間”的狀語連用,表示動作或狀態(tài)從某一時刻開始，一直持續(xù)到現(xiàn)在。如：I haven't seen him for two years. 但是，像come, arrive, buy等終止性動詞不能與表示“一段時間”的狀語連用。要用，必須改為“be（在）”等延續(xù)性動詞來表述?，F(xiàn)歸納總結一下由非延續(xù)性動詞到延續(xù)性動詞的轉換：
arrive→be here begin(start)→be on
die →be dead come back→be back
leave →be away fall ill(sick,asleep)→be ill(sick,asleep)
get up→be up go out →be out
finish →be over put on →wear 或be on
open →be open join →be in或 be a member of…
close →be closed go to school→be a student
borrow →keep buy →have
catch(a cold)→ have(a cold) get to know →know
begin to study→study come to work→work等
如：He has been a soldier for three years.他參軍三年了。
His father has been dead for two years.他父親去世二年了。
The film has been on for ten minutes.電影已開始十分鐘了。
We have studied English for three years. 我們（開始）學英語已三年了。
