if引导的条件状语从句的结构
对将来进行假设主要有三种情况：
1,if条件从句是谓语用动词过去式,则后面的主句是情态动词加动词原形
例：If I came tomorrow ,we would have the meeting.
2,if条件从句是were to形式,则后面的主句也是情态动词加动词原形
例：If it were to rain tomorrow ,the meting would be put off.
3,if条件从句是should do形式,则后面的主句可以用情态动词加动词原形,也可以用祈使句（但注意,祈使句仅适用于前面从句是should do形式的）
例：If it should rain tomorrow,don't expect me./I would not go there.
扩展资料
对现在事实相反的虚拟语气
1、表示与现在事实相反的情况
从句：If+主语+动词一般过去时(Be动词用were)
主句：主语+ should/would/might/could+do：
If I were you，I would take an umbrella.
如果我是你，我会带把伞。（事实：我不可能是你）
If I knew his telephone number，I would tell you.
如果我知道他的电话号码，我就会告诉你。（事实：不知道）
If there were no air or water，there would be no living things on the earth.
如果没有水和空气，地球上就不会有生物。(事实：地球上既有空气也有水)
If I had any money with me，I could lend you some.
如果我带钱了，我就会借给你些。（事实：没有带钱）
If he studied harder，he might pass the exam.
如果他再努力些，就能通过考试了。（事实：没有努力）
If I were you， I'd wear a shirt and tie.
如果我是你，我就会穿衬衫和领带。(事实：我并不是你）
2、表示与过去事实相反的情况
从句：If+主语+had+done
主句：主语+should/would/might/could+have done
eg：If I had got there earlier，I should/would have met her.
如果我早到那儿，我就会见到她。（事实：去晚了）
If he had taken my advice，he would not have made such a mistake.
如果他听我的劝告的话，就不会犯这样的错误了。（事实：没有听我的话）
参考资料：百度百科-虚拟语气
