1. 由but for, with, otherwise等介词短语引导的句中。此类介词短语多引导含条件意味的虚拟语气。如：
①The ship would have sunk with all on board but for the efforts of the captain. 要没有船长的努力，那这只船及船上所有的人都沉没了。
②With your help, I would do the experiment well. 有了你的帮助，我会把实验好好地做的。
③I am really busy, otherwise I would certainly go with you. 我真得好忙，否则我当然跟你一起去了。
2. if only引导的特殊句式。此种句式相当于感叹句，表示“要是…… 就好了!”，表示现在的愿望用过去时;表示过去的愿望用过去完成时;表示将来的愿望用过去时或过去将来时。如：
①If only I were a bird!要是我是一只鸟就好了。
②If only we had listened to his advice!我们要是听了他的建议就好了。
③If only it would stop raining!但愿雨能停下来!
