没错,不能用He has left since three years ago,只能改用He has been away since three years ago.
无论是“ for + 一段时间 ” 还是 “since + （过去的）时刻 ”,只要句子是肯定句,就要求谓语动词必须能够延续,而不能是瞬间性动词.
如果句子是否定句,就没有这个限制了.
如：I haven't seen/met you since ten years ago.
= I haven't seen/met you for ten years.
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