11.12 比较since和for

　　Since 用来说明动作起始时间，for用来说明动作延续时间长度。例如：

　　I have lived here for more than twenty years.我住在这儿二十多年了。

　　I have lived here since I was born. 我从出生起就住在这儿了。

　　注意：并非有for 作为时间状语的句子都用现在完成时。

　　I worked here for more than twenty years. （我现在已不在这里工作。）

　　I have worked here for many years.（现在我仍在这里工作。）

　　注意：用句型转换的方法，很容易排除非延续动词在有for/since结构的完成时中的误用。

　　1）（对） Tom has studied Russian for three years. = Tom began to study Russian three years ago, and is still studying it now.

　　2）（错） Harry has got married for six years. = Harry began to get married six years ago, and is still getting married now.

　　显然，第二句不对，它应改为 Harry got married six years ago. 或 Harry has been married for six years.

　　11.13 since的四种用法

　　1） since +过去一个时间点（如具体的年、月、日期、钟点、1980, last month, half past six）。例如：

　　I have been here since 1989. 1989起，我一直在这儿。

　　2） since +一段时间+ ago。例如：

　　I have been here since five months ago. 我在这儿，已经有五个月了。

　　3） since +从句。例如：

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

　　Great changes have taken place since we were here. 我们走后，变化可大了。

　　4） It is +一段时间+ since从句。例如：

　　It is two years since I became a postgraduate student. 我考上研究生有两年了。

　　11.14 延续动词与瞬间动词

　　1） 用于完成时的区别

　　延续动词表示经验、经历； 瞬间动词表示行为的结果，不能与表示段的时间状语连用。例如：

　　He has completed the work.　　他已完成了那项工作。 （表结果）

　　I've known him since then.　　 我从那时起就认识他了。（表经历）

　　2） 用于till / until从句的差异

　　延续动词用于肯定句，表示"做……直到……" 瞬间动词用于否定句，表示"到……，才……"。例如：

　　He didn't come back until ten o'clock. 他到10 点才回来。

　　He slept until ten o'clock.　 他一直睡到10点。

　　典型例题

　　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. 首先本题后句强调对现在的影响，我知道她的模样，你不用描述。再次，several times告知为反复发生的动作，因此用现在完成时。

　　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. 等待的动作由过去开始，持续到现在，应用现在完成时。

　　11.15 过去完成时

　　1） 概念：表示过去的过去

　　----|----------|--------|----> 其构成是had +过去分词构成。

　　那时以前　 那时　现在

　　2） 用法

　　a. 在told, said, knew, heard, thought等动词后的宾语从句。例如：

　　She said （that）she had never been to Paris. 她告诉我她曾去过巴黎。

　　b. 状语从句

　　在过去不同时间发生的两个动作中，发生在先，用过去完成时；发生在后，用一般过去时。例如：

　　When the police arrived, the thieves had run away. 警察到达时，小偷们早就跑了。

　　c. 表示意向的动词，如hope, wish, expect, think, intend, mean, suppose等，用过去完成时表示"原本…，未能…"。例如：

　　We had hoped that you would come, but you didn't. 那时我们希望你能来，但是你没有来。

　　3）　过去完成时的时间状语before, by, until , when, after, once, as soon as。例如：

　　He said that he had learned some English before. 他说过他以前学过一些英语。

　　By the time he was twelve, Edison had began to make a living by himself. 到了十二岁那年，爱迪生开始自己谋生。