英语中的虚拟语气，是说话者用来表示假设，或难以实现的情况，而非客观存在的事实，所陈述的是一个条件，不一定是事实，甚至完全与事实相反。此外如需表达主观愿望或某种强烈的感情时，也可用虚拟语气。虚拟语气通过谓语动词的特殊形式来表示。实际上主要是三种时态：与现在事实相反，与过去事实相反，与将来事实相反。

（1）与现在事实相反

若与现在事实相反，虚拟语气的条件从句的谓语用过去式(be通常用were)，主句谓语用“should (would, could, might)+动词原形”。如：

If you took a taxi，you'd get there quicker.

If I knew her number I could ring her up.

（2）与过去事实相反

若与过去事实相反，虚拟语气的条件从句的谓语用过去完成时(had+过去分词)，主句谓语用“should (would, could, might)+have+过去分词”。如：

If I'd left sooner，I'd have been on time.

If we had found him earlier we could have saved his life.