The structure of the Present Perfect is: have/has done
现在完成时结构：have/has done
●When we talk about something that happened in the past, but we don’t specify precisely when it happened, we use the Present Perfect.
●表示某事发生在过去，但并未给出具体的发生时间，用现在完成时。
★A French yachtsman has broken the record for sailing around the world single-handed.
★一名法国男子驾帆船环球航行，打破了单人环球航行的记录。
★Research has shown that cycling can help patients overcome their illness.
★研究表明，骑自行车有助于患者战胜疾病。
●When we usethe Present Perfect, it suggests that something that happened in the past affects the situation that exists now.
●当表示某件发生在过去的事情对现在造成了影响，用现在完成时。
★We can’t go ahead with the meeting, because very few people have shown any interest.
★我们不能继续开会了，因为好多人都不感兴趣。
★With this promotion, I feel that I’ve reached a turning point in my career.
★此次晋升让我觉得我的职业生涯出现了转折点。
●When we talk about how long an existing situation has lasted, even if we don’t give a precise length of time, we usethe Present Perfect.
●当表示某种现有情况持续了多长时间，即便没有给出具体的持续时间，用现在完成时。
★They’ve grown such a lot since we last saw them.
★自我们上次见到他们后，他们长高了许多。
★Prices have fallen sharply over the last five months.
★在过去的五个月里，价格出现断崖式下跌。
●We often usethe Present Perfect to say that an action or event has been repeated a number of times up to now and the action or event might happen again.
●现在完成时可用来表示从过去某个时间点到现在反复发生的事件，而且这个事件可能还会再次发生。
★They have been to Chile three times.
★他们已经去过智利三次了。
★She has been seen to meet with Andy at least four times in the past three years.
★在过去的三年里，有人看见他至少与安迪见过四次面。
★We have had four tests so far this term.
★到目前为止，本学期我们已经进行了四次考试。
●Some time adverbs like lately, already, so far, yet, still, up to now, up to the present, that connect the past to the present are often used with the Present Perfect.
●一些时间副词，如lately, already, so far, yet, still, up to now, up to the present经常用于现在完成时。
★Have you seen Cathy lately?
★你最近见过凯西吗？
★Have you finished your homework yet? I still haven’t finished mine.
★你写完作业了吗？我还没写完。
★So far we have only discussed the first five chapters.
★到现在为止，我们只讨论了前五章。
●SINCE+过去的某个时间点，表示某事从过去的某个时间点一直持续到现在，主句用现在完成时。
★My parents have lived in London since 1985.
★我的父母自1985年以来一直住在伦敦。
★I have been in this city since last May.
★自去年五月开始，我就一直生活在这个城市。
●SINCE+从句，SINCE从句用一般过去时，主句用现在完成时。
★She has been a teacher since she left university.
★从毕业后，她一直在当老师。
★Since Cathy became president, both taxes and unemployment have increased.
★自凯西当上主席后，税收和失业均呈上涨态势。
★Since the eruption started, all the villages on the slopes of the volcano have been evacuated.
★自火山爆发以来，火山坡上的所有村庄都被疏散了。
●We can use FORwith both the Past Simpleandthe Present Perfect.
●FOR+时间段，既可用一般过去时，也可用现在完成时。
★I was stuck in traffic for hours. (the traffic is over)
★我被堵了好几个小时。
★Andy has worked here for 10 years. (he is still working here)
★安迪已经在这里工作了10年。
★Today is the 14th of June. I bought this book two weeks ago. I have had this book since the first of June. I have had this book for two weeks now.
★今天是6月14日，两个星期前我买了这本书。6月1日那天我就拥有了这本书，如今我拥有这本书已经两个星期了。
★Dinosaurs have been extinct since about 65 million years ago.
恐龙在约6,500万年前就已经灭绝了。
Dinosaurs have been extinct for about 65 million years.
恐龙灭绝距今已约6,500万年。
The Past Simple (I did)一般过去时
●一般过去时表示在过去的某个特定的时间发生了某事，而且这件事的开始和结束都发生在过去。
★She arrived at Kennedy Airport at two o’clock this morning.
★她于今天凌晨两点到达了肯尼迪机场。
★Jane left just a few minutes ago.
★简几分钟前才离开。
★Quite early in the negotiations, they agreed to lower the prices.
★在谈判的早期，他们同意降低价格。
★The Earth formed about 4,500 million years ago.
★地球形成于约45亿年前。
●一般过去时用来表示在过去一段时间内存在的某种情况，但现在已经不存在了。
★The pharaohs ruled Egypt for thousands of years.
★法老统治埃及长达数千年之久。
★In his twenties, Andy spent many years travelling around Spain.
★安迪20多岁的时候，花了很多年时间在西班牙旅行。
●If we are interested in when a present situation began rather than how long it has been going on for, we use the Past Simple. Compare:
●如若想表达现在的某种情况于何时开始的，而非已经持续了多长时间，用一般过去时，试比较：
★I started to get the pains three weeks ago.
我三周前就开始痛了。
I’ve had the pains for three weeks now.
我现在已经痛了三个星期了。
★When did you arrive in Britain?
你什么时候到的英国？
How long have you been in Britain?
你在英国呆了多久了？
●表示过去的时间副词a month ago、at 3 o’clock、last week、then、yesterday、just now、at that time、once使用一般过去时。
★Cathy died, at the age of 86, in 1964.
★凯西在1964年去世，享年86岁。
★I knew a man once who had fifteen cats.
★我曾经认识一个人，他养了15只猫。
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