11.12 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。例如：

　　I have lived here for more than twenty years.我住在這兒二十多年了。

　　I have lived here since I was born. 我從出生起就住在這兒了。

　　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。）

　　注意：用句型轉(zhuǎn)換的方法，很容易排除非延續(xù)動詞在有for/since結(jié)構(gòu)的完成時中的誤用。

　　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(yīng)改為 Harry got married six years ago. 或 Harry has been married for six years.

　　11.13 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）。例如：

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

　　2） since +一段時間+ ago。例如：

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

　　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。

　　11.14 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(jié)果，不能與表示段的時間狀語連用。例如：

　　He has completed the work.　　他已完成了那項工作。 （表結(jié)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……"。例如：

　　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(fù)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(yīng)用現(xiàn)在完成時。

　　11.15 過去完成時

　　1） 概念：表示過去的過去

　　----|----------|--------|----> 其構(gòu)成是had +過去分詞構(gòu)成。

　　那時以前　 那時　現(xiàn)在

　　2） 用法

　　a. 在told, said, knew, heard, thought等動詞后的賓語從句。例如：

　　She said （that）she had never been to Paris. 她告訴我她曾去過巴黎。

　　b. 狀語從句

　　在過去不同時間發(fā)生的兩個動作中，發(fā)生在先，用過去完成時；發(fā)生在后，用一般過去時。例如：

　　When the police arrived, the thieves had run away. 警察到達時，小偷們早就跑了。

　　c. 表示意向的動詞，如hope, wish, expect, think, intend, mean, suppose等，用過去完成時表示"原本…，未能…"。例如：

　　We had hoped that you would come, but you didn't. 那時我們希望你能來，但是你沒有來。

　　3）　過去完成時的時間狀語before, by, until , when, after, once, as soon as。例如：

　　He said that he had learned some English before. 他說過他以前學過一些英語。

　　By the time he was twelve, Edison had began to make a living by himself. 到了十二歲那年，愛迪生開始自己謀生。