but for 要不是，如果没有
if only... 要是…就好了
needn't have done 表示：过去不必做其事，但事实上做了
must have done 一定是…（对过去非常肯定的猜测）
can't have done 不可能
couldn't have done 不可能已经...
could have done 本来应该
should have done 本来应该
shouldn't have done 本来不应该
would have done 本来会
wouldn't have done 本来不会
might have done 可能已经
might not have done 可能没有
急用
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