时间段 + ago——用于完成时的三种情况
通常ago与过去时（一般过去时、过去进行时等）连用，但并非仅仅如此。
我们也看到了ago用于完成时的三种情形。
第一：时间段+ ago用于“过去完成时”
第二：时间段+ ago用于“情态动词+ 现在完成（进行）时”（表推断）
第三：时间段+ ago用于“现在完成时”
附：时间段+ ago用于“过去进行时”例：
第一：时间段+ ago用于“过去完成时”
《薄冰英语用法问答1144例》第412页
I had thought that he had died at least twenty years ago.
这里用过去完成时had died是因为它发生在主句谓语动词had thought之前。
虽然that从句可以译作“他至少已死了20年”，但并无延续性。
《英语常用词疑难用法手册》（陈用仪编著）第347页had + pp
本来，表示事件的现在完成时不可以同表示确切时间点的时间状语连用（不可以说He has left at 4:00 p.m.），但是过去完成时却可以说He had left at 4:00 p.m.（为He had left既可以是He has left的向过去移动一格，也可以是He left的向过去移动一格，作为后者可以加上明确时间）。【例如】
The present storm had been born five days agoin the ice of the Colorado Mountains. (Arthur Hailey, Airport, p.5)
目前这场暴风雪是五天前从科罗拉多山脉上的结冰那里发源的。
第二：时间段+ ago用于“情态动词+ 现在完成（进行）时”（表推断）
《英汉多功能学习词典》第36页ago词条
ago表示从现在算起一段时间以前，和过去式连用，不能和现在完成式连用。
不过，从现在推断过去的事情时，可以用完成时：
That must have been years ago. 那肯定已是几年前的事情了。
《朗文高级英语应试词典》第46页ago词条
He should have finished at university long ago, but he kept taking extra courses.
他大学早就该毕业了，但是他不停地选修额外课程。
《现代英语用法词典》（张道真主编修订版）第34页ago词条
He ought to have been here long ago.
《新时代英汉大词典》第52页ago词条
The liquour must have been standing there from long ago, awaiting some joyous occasion.
那酒想必早就摆在那儿了，等着某个喜庆场合时好喝。
2015年高考英语，湖南卷第29题，原题为：
It is a truly delightful place, ___________ looks the same as it must have done 100 years ago with its winding streets and pretty cottages.
A: as B: where C: that D: which
2010年高考英语，安徽卷第32题，原题为：
Jack described his father, who _________ a brave boy many years ago, as a strong-willed man.
A: would be B: would have been C: must be D: must have been
《新概念英语》第三册 Lesson 58 A Spot of Bother一点儿小麻烦
At least sorting out the things she should have discarded years ago was now being made easier for her.
第三：时间段+ ago用于“现在完成时”
《薄冰英语用法问答1144例》第411页
我们知道，现在完成时可后接表笼统的过去时间的状语，如：
I have ceased to believe that long ago.
不少语法家们都说long ago在此是一种添补之词，是事后想到的一种说法。但这只限于表笼统的过去时间的状语的说法看来已被突破，现在也可用表具体的过去时间状语了We have already discussed this affair last night即是一例。再如：
The five-thirty train has been in and gone half an hour ago.
请注意这种用作添补之词的时间状语理应置于句末。
《葛传槼英语惯用法词典》ago词条
一般说来，用了ago不可用完成时态，如可以说She saw him a week ago，不可说She has seen him a week ago。但用了助动词或用在分词、动名词或不定式的形式里便可以用完成时态。例如：
She must have seen him a week ago.
She may have seen him a week ago.
She cannot have seen him a week ago.
Having seen him a week ago, she is always thinking of him.
She speaks of having seen him a week ago.
She seems to have seen him a week ago.
即使不用助动词，也不用在分词、动名词或不定式的形式里，偶尔也有用完成时态而用ago的。例如：I have long ago given up that habit.这里语气重在“早已”。
附：
时间段+ ago用于“过去进行时”例：
《新时代英汉大词典》第52页ago词条
You were doing it a moment ago, before you spoke to me.
刚才你还在做那件事儿呢，就在你跟我说话前那会儿。
《牛津英语用法指南》（第三版）第47页ago词条
Where’s Mike? ~ He was workingoutsideten minutes ago.
迈克在哪儿？~ 十分钟以前，他还在外面干活儿呢。
2008年高考英语，辽宁卷阅读理解B 第三自然段第一句话：
A few weeks ago, Wesley Autrey was waiting at a Harlem subway station with his two little girls when he saw a man fall into the path of a train.
2017年3月25日