他不可能给我写信了.can't have done 不可能,表示对过去的猜测.or I'd have got the letter by now.= or I would have got the letter by now否则的话,我现在已经收到信了.否则的话,也就是 如果她给我写了信的话.这...can't和 by now 不应该是与现在相反吗 谢谢by now适合的时态是现在完成时， 动作发生在过去。