过去将来完成时 Past Future Perfect

表示在过去的时间看来，在将来某一时刻之前就已经完成的动作；也可用于虚拟语气表示“本该做但未做”。

• 结构公式：主语 + would have + done
• 否定式：主语 + would not have (wouldn't have) + done
• 疑问式：Would + 主语 + have + done...?
• I said I would have done my homework before you opened the door. —— 我说过在你开门之前我就会做好功课。
• He believed that by the next month he would have finished the book. —— 他深信到下个月他就已经读完那本书了。
• If you had helped me, I would have completed it earlier. —— 如果你当时帮了我，我本来可以更早完成的。（虚拟语气）

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