一、规则：在通常情况下 ago 只与过去时连用，而不与现在完成时连用。因为ago指的是多久以前的过去时间，而现在完成时表示的动作虽然发生在过去，但它强调的是与现在的联系或对现在的影响，因此两者不能用在一起。如：我两个月前见过他。误：I have seen him two months ago. 正：I saw him two months ago.
二、特例：但是，有时为了行文的需要或使句子生动，ago可用于类似以下这样的句子：I had thought that he had died at least 20 years ago. 我原以为他至少在< ago. years 20 least at died had he that>20年前就已经去世了。
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