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虛擬語氣是動詞的一種特殊形式,用來表達說話人的意愿。請求,設想,等未能或不可能實現的事實,或說話人看來實現可能性很小的情況。做這類題時首先要弄清楚虛擬語氣的各種句型,即由if引導的虛擬條件句,省略if的倒裝形式,可跟虛擬語氣的賓語從句,同位語從句,表語從句的句型,以及表示祝愿或感嘆的句子和由without, but for等介詞引導的短語或句子。 (1)虛擬語氣現在時。 用以表示與現在事實相反的假設或現在實現可能性不大的情況。 條件從句：動詞過去式(be用were) 主 句：Would/should/could/might + 動詞原形 Eg。 If I were a bird, I would fly to you。If it were not for their help, we should be in a very difficult position。 (2) 虛擬語氣過去式。 表示與過去事實完全相反的假設。 條件從句：had + 過去分詞 主 句：would/should/could/might + have +過去分詞 Eg。 If we had started earlier, we should not have missed the train。 If she hadn’t been ill, she might have come。 (3)虛擬語氣將來式。 表與將來情形相反的假設或極少有可能實現的情況。 條件從句：動詞過去式或should +動詞原形或were to +動詞原形 主 句：would/should/could/might +動詞原形 Eg。 If Professor Li should have time tomorrow, we could ask him some questions。 If there should be no air, there would be no living things。 。