PETS－2语法讲解(2)

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。
　　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。)
　　小窍门：　当现在完成时+一段时间，这一结构中，我们用下面的公式转化，很容易就能排除非延续动词在完成时中的误使。
　　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.
　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)。
　　I have been here since 1989.
　　2) since +一段时间+ ago
　　I have been here since five months ago.
　　3) since +从句　
　　Great changes have taken place since you left.
　　Considerable time has elapsed since we have been here.
　　4) It is +一段时间+ since从句
　　It is two years since I became a postgraduate student.
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。
　　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……"
　　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。