时(tense)是个语法范畴，它是表示时间区别的动词形式。英语动词只有“现在时”和“过去时”，而没有“将来时”(在英语中，表示“将来”手段多种多样，但没有一种独特的、能与“现在时”和“过去时”平起平坐的专一表示“将来”的动词形式----“将来时”)。
体(aspect)也是一个语法范畴，它表示动作或过程在一定时间内处于何种状态的动词形式。英语有进行体(progressive aspect)和完成体(perfective aspect)。进行体是由助动词be的一定形式加主动词的-ing分词构成;完成体由助动词have的一定形式加主动词的-ed分词构成。
现在时和过去时既可以单独使用，也可以和进行体或完成体结合使用，也可以同时与完成体和进行体结合使用。这样，英语的限定动词词组便有8种时、体形式。它们分别是：一般现在时(simple present)、一般过去时(simple past)、现在进行体(present progressive)、过去进行体(past progressive)、现在完成体(present perfective)、过去完成体(past perfective)、现在完成进行体(present perfective progressive)、过去完成进行体(past perfective progressive)。在这一章中，我们单独挑出完成体来加以详述。
1. 必须使用完成体的结构
1)It (This, This evening, yesterday...) is (was, will be) first (second, third...) time (day, month…)…结构中的分句，要求用完成体。
Is this the first time you've been to Beijing?
This was the first time he had been to Beijing.
This is the eighth month that I have been out of work..
This was the eighth month that I had been out of work.
This is the second time that the goods produced by our factory have been shown in the International Exhibition.
This was the second time that the goods produced by our factory had been shown in the International Exhibition.
2)在no sooner…than, hardly/barely/scarcely…when,等的句型中，主句要用过去完成体。
He had no sooner seen me than he left the room.
No sooner had he seen me than he left the room.
The helicopter had hardly landed when the waiting crowd ran toward it.
Scarcely had I seen the lightning when I heard a clap of thunder.
3)将来完成体用来表示在将来某一时间以前已经完成或一直持续的动作。经常与before+将来时间或by+将来时间连用，也可与before或by the time引导的现在时的分句连用。
I will have finished all the work by the time you are back this evening.
I am sure he will have left Paris by this time tomorrow.
I hope we will have got all the information before you come tomorrow.
By the time you get to New York, I _______for London. (2002年1月)
A) would be leaving B) am leaving
C) have already left D) shall have left
本题时间状语为by+将来时间，考察将来完成体用法，应选择D)。
By the time he arrives in Beijing, we ________here for two days.(2001年6月)
A) have been staying B) have stayed
C) shall stay D) will have stayed
将来完成体用来可以表示在将来某一时间以前一直持续的动作，本句话的意思是：我们将在这里呆两天，因此谓语动词用将来完成体，答案为D)。
2. 现在完成体与现在完成进行体
现在完成进行体兼有现在完成体和现在进行体二者基本特点。由于它有现在完成体的特点，所以它可以表示某一动作对现在产生的结果或影响。由于它有现在进行体的特点，所以它也可以表示某一动作的延续性、临时性、重复性、生动性乃至感情色彩。
1)现在完成进行体和现在完成体皆可表示动作对现在产生的结果，但前者所表示的结果是直接的，而后者所表示的则是最后的结果。
We have been cleaning the classroom.(a)
We have cleaned the classroom.(b)
(a)句可译为“我们打扫教室来着”。其直接结果可能是：我们身上都是灰。(b)句可以译为“我们把教室打扫过了”。其结果是：现在教室很清洁，可以用了。另外(a)句表示教室刚刚打扫过，(b)句则可能表示教室是昨天打扫的。再看下面两个句子：
Be careful! John has been painting the door.(a)
John has painted the door.(b)
(a)句表示约翰刚刚把门油漆过，现在油漆还未干，所以你要小心。(b)句则无此含义，油漆可能已干了。
2)现在完成进行体有时有延续性;现在完成体往往没有。
They have been widening the road.(a)
They have widened the road.(b)
(a)句的意思是他们在加宽马路，但尚未完工。(b)句的意思则是已完工了。有时现在完成体有延续性(如一些属于持续体的动词)，但无临时性质。
Mr. Smith has been living in London since 1978.(a)
Mr. Smith has lived in London since 1979.(b)
(a)句有“史密斯先生在伦敦久居”的含义，(b)句则没有。
3)但现在完成进行体并不总是具有临时的性质。
My mother has been teaching English for twenty years.(a)
My mother has taught English for twenty years.(b)
(a)句在此并无临时性质，但较口语化。(b)句则较为正式。另外，(a)句表示动作现在仍在继续.并将延续下去;(b)句的动作是否延续下去，须由上下文决定，但在一般情况下都是延续下去的。
4) 在完成进行体往往表示动作在重复;现在完成体则常常不带重复性。
Have you been meeting her lately?(a)
Have you met her lately?(b)
(a)句有“经常相会”之意，(b)句则没有。(b)句如与often，every day等时间状语连用，当然也表示动作在重复。
5) 在否定结构中，现在完成体所否定的是谓语动词;现在完成进行体所否定的是状语。
He hasn't been speaking since three o'clock.(a)
He hasn't spoken since three o'clock.(b)
(a)句中所否定的不是has been speaking，而是 since three o'clock，其结构等于 He has been speaking not since three o'clock but since half past three.这是因为现在完成进行体本身是没有否定结构的原故。(b)句用的是现在完成体，而现在完成体是有否定结构的，所以(b)句中所否定的自然是has spoken，意即“从三点起他一直沉默不语，未发一言”。
3.情态动词 + 行为动词完成体
1) must + have + V-ed
指现在对过去已发生的事或可能出现的情况进行推断和猜测，表示推测过去某事“一定”发生了。因此，You must see her last night的说法是错误的。
She must have met a ghost yesterday. 她昨天一定碰见过鬼了。
He must have been there last week.他上周一定在那儿。
His score on the English test is the highest in the class; he must have studied last night.他这次英语测试的成绩全班最高。他昨晚一定在好好学习。
2) can’t/couldn’t + have + V-ed
can’t/couldn’t + have + V-ed为must + have + V-ed的否定形式，指现在对过去已发生的事或可能出现的情况进行推断和猜测，表示过去不可能发生某事。
They can’t have come here in their car yesterday, for it was then under repair.他们昨天不可能开车到这里，因为那时他们的车还正在修理当中。
The room is in a mess; it can’t/couldn’t have been cleaned.
3) may/might + have + V-ed
表示推测过去某事“也许”发生了。
He may/might have chosen another career if he had had enough money to further his education.如果他那时有足够的钱继续受教育，他也许会选择另一个职业。
I might have fulfilled the work earlier. 我本可以早一些完成这件工作的。
4) ought to/should + have + V-ed和ought not/shouldn’t + have + V-ed
对以发生的情况表示“不满”、“责备”，分别表示“本应该…”和“本不应该”
I wonder why they haven’t arrived yet. I told them how to get there, but perhaps I ought/should give them a map.(含有“后悔”或“自责”之意)
You ought to have told me that yesterday.你早在昨天就该把那件事告诉我了。
5) needn’t + have + V-ed
表示过去做了某事，但没有做的必要，意为“本没必要…”。
You needn’t have hurried.
She needn’t have come in person--- a letter would have been enough.
You needn’t have done all those calculations. We have a computer to do that sort of thing.
4.用在was/were, planned, intended, hoped, wished + 不定式完成体
表示事实上并未实现的计划或行为。
We were to have started at 8 last night, but it snowed heavily.
They intended to have gone camping, but they later cancelled it.
I hoped to have finished it last week, but I was too busy.
From:https://www.zgghmh.com/CET-4/20000695.asp