赖世雄教你学英语语法
虚拟语气
2-2：用法及注意事项
使用与现在事实相反的虚拟语气时，要注意下列要点；
1.本虚拟语气旨在表示与现在事实相反的情况。换言之，一见到此类过去时句型出现时，我们就可确知事实并非如此。
例：if i were rich, i would help you.
= as i am not rich, i can't help you.
（如果我很有钱，我就会帮助你。——因为我不是很有钱，所以免法帮助你。）
they might finish the work befor dark, if we helped them.
= in fact, it is impossible that we will help them, so they can't finish the work before dark.
（如果我们帮他们的话,天黑前他们就可以做完工作了。——其实，我们不可能帮助他们，所以天黑前他们也做不完工作。）
2.不论主语为第几人称，if从句中的be动词，均使用were
例：if you were in my shoes, what would you do?(o)
（如果你站在我的立场，你会怎么做？）
if i was you, i wouldn't do it.(x)
---- if i were you, i wouldn't do it.(o)
（如果我是你,我不会做这事。）
3.主句中的助动词一定为过去时
助动词过去时will—would，may－might，can－could, shall－should或ought to。但must（必须）只能表示现在或将来的状况，故只用于纯条件的虚拟语气，而绝不可用于表与现在事实或过去事实相反的虚拟语气中。
例：if you did it for me, i should be very happy.(o)
（如果你为了我这样做，我会很高兴。）
if i could fly, i would fly to you.
（如果我能飞，我会飞向你。）
if you had money, you must help the poor.(x)
---- if you had money, you ought to help the poor.(o)
---- if you had money, you should help the poor.(o)
（如果你有钱，你应该帮助穷人。）
或：if you have money, you (must/should/ought to) help the poor.(o)