核心问题
I told them after you left.
after 从句的动作发生在主句动作之前,为何不用过去完成时 had left?
词典解答
由于 after, as soon as, when 等连词能清楚地表达从句动作与主句动作的先后关系, 所以可以用一般过去时来代替过去完成时: I told them after you left. = I told them after you had left.
再如：
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As soon as I got home my brother went out. 我刚一到家，我兄弟就走了。
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As soon as I (had) put the phone down it rang again. 我刚把电话放下, 就又响了。具有“紧接……之后”含义的 when, 也能明确表达时间的先后关系, 所以, when 从句也可以使用一般过去时来代替过去完成时:
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I ate my lunch when Sandra came back from her shopping. 我是在桑德勒购物回来之后吃的午饭。
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When I put the cat out, it ran off into the bushes in disgust. 我把那只猫放出去，它就厌恶地跑开，钻进了灌木丛。
past perfect tense (过去完成时)
过去完成时表达未实现的愿望 见 expect , mean , intended to do