should have done 表示过去本该做而实际未做，与过去事实相反。“She should have arrived here by now.”这句是不成立的，时间不一致。 应该是：“She should have arrived here by then.”“到那时她本该到那里了，而实际未到。与过去事实相反”She should be here by now. 句子是可以的，意思是“到现在为止她应该到这儿了，（暗含她现在还没到，与现在事实相反）”She should know the news by now. “到现在为止她应该知道知道这个消息了”（暗含她现在可能还不知道，与现在事实相反）”She knew?the news before she read the newspaper . 是错句She should have known the news before she read the newspaper . 是对的。“在她（过去）读报纸之前，她本应该已经知道这条消息。（而实际不知道，与过去事实相反）”这些都是虚拟语气范畴的句子，与表面意思是相反的。