ago何时用于完成时

ago和一般过去时连用，表示以现在为起点的“以前”，一般不用于完成时态。

但在以下情况下是可以用于完成时：

1.ago与情态动词加不定式完成时连用，表示现在对过去已发生的某事的推测或表示现在认为某事本应该发生。例如：

He must have left a week ago.他准是在一周前就离开了。

Faber must have seen her a month ago.费伯一个月前一定见过她。

She ought to have been here long ago.她早就应当到这里了。

I seem to have read the book two years ago.

我似乎两年前就读过这本书。

2.在since+一段时间+ago结构的句子里，可以用完成时态，此时可以理解为介词since+名词短语结构即介宾结构：

He’s been there since two years ago.

他自从两年前就已经在那儿了。

I have been here since five months ago.

我在这儿已经有五个月了。

I haven’t heard from her since ten years ago.

我自从10年前以来就没有收到她的来信了。

I have studied in Peking University since 2 years ago.

我从两年前起，就在北大上学。

I’ve kept up a friendship with a girl whom I was at school since twenty years ago.

我和一个我学生时代在一起的女孩保持了20年的友谊。

顺便说一下 before表示以过去某时间为起点的“以前”，也可泛指“从前”，用于在完成时、过去完成时或过去时。例如：

I have read it before.以前我读过它。

I didn’t know that before.以前我不知道那件事。

He told me that he had handed in his exercise book two days before.

他告诉我他两天前已经交了作业本。

比较：

Herbert said that his mother had died five years before.

赫伯特说他的母亲五年前去世了。(距他说话时五年了)

His mother died five years ago.

他的母亲五年前去世了。(距现在五年）