在英语中，条件句分为真实条件句和虚拟条件句。真实条件句基于可能的情况，主句不用虚拟语气，如：“If I have time, I will go.”虚拟条件句则描绘一种假设，与事实相反，需使用虚拟语气，如：“If I were you, I would go.”
虚拟语气在条件句中的使用根据所指时间有所不同。与现在事实相反，主句用“would/should/could/might + V原”，条件句用动词过去式或be动词were，例如：“If I were you, I should study English.”与过去事实相反，主句用“would/should/could/might + have done”，条件句用过去完成时，如：“If you had taken my advice, you would not have failed in the test.”
对于将来事实相反，主句可用“would/should/could/might + V原”或“should + V原”或“were + to do”，如：“If you came tomorrow, we would have the meeting.”虚拟语气的时态调整需要根据实际语境进行。
特别说明，would在虚拟语气中可能表示结果、过去习惯或可能性，如：“If you tried again, you would succeed.”(would表结果)；而might则强调可能性：“If you tried again, you might succeed.”(might表可能)。错综时间条件句和否定形式的虚拟条件句，如：“If it hadn’t been for your assistance, we wouldn’t have succeeded.”也需注意调整时态和表达方式。
最后，虚拟条件句有时会出现倒装，如：“Were I in school again, I would work harder.”这种形式强调了假设条件。