假设虚拟语气
当我们使用 as if 或 as though 引导的虚拟语气时，我们通常是在描述一个虚构的或不太可能实现的情况。As if 和 As though 结构：主句 + as if/though从句（虚拟语气）。例如：
- 现在虚拟语气：
- She looks as if she is about to cry. -> 她看起来好像快要哭了。
- He talks as though he knows everything. -> 他说话好像什么都知道。
- 过去虚拟语气：
- She acted as if she had seen a ghost. -> 她的行为好像看到了鬼。
- He spoke as though he had been there before. -> 他说话好像以前去过那里。
- 将来虚拟语气：
- She talks as if she were going to win the lottery. -> 她说话好像要中彩票一样。
- He acts as though he would be the next president. -> 他的行为好像会成为下一任总统一样。
if only引导的虚拟语气
if only引起的从句表示一种不太可能实现的愿望或遗憾，因而常用虚拟语气。从句的谓语动词用过去式或过去完成时，表示现在或过去的情况。
- If only I had more money. 要是我有更多的钱就好了。（现在）
- If only he had listened to me. 要是他听了我的话就好了。（过去）
- If only she would come back. 要是她能回来就好了。（将来）
用于定语从句
在表示“早该”的句型中，用于修饰时间的从句谓语动词用过去式或“should+动词原形”，而且should不能省略。例如：
- It is high time that he went to bed. 他早该睡觉了。
- It is about time that we should leave. 我们早该走了。
- It is high time that you should be more careful. 你早该更小心了。
含蓄条件句
含蓄条件句，是指在表示虚拟的句子中，条件从句没有明确表达，而是隐含在短语或上下文中，或者已经成为固定用法的句子。
- But for the rain, we would have gone out. 要不是下雨，我们就会出去的。（暗含在分词短语 but for the rain 中）
- I could do it better with more time. 如果有更多的时间，我能做得更好。（暗含在介词短语 with more time 中）
- We would have succeeded. 我们本来是会成功的。(可能暗含if we had kept trying)
- He might have been a great writer. 他本来可能是个伟大的作家。（暗含在上下文中，可能是 if he had worked hard）
- I should be grateful if you could help me. 如果你能帮助我，我会很感激。（已经成为固定用法）
- I would like to come. 我愿意来。（虚拟式已变成习惯说法）