eg：If I were you，I would take an umbrella.
eg：If I had got there earlier，I should/would have met her.
从句：①if+主语+were to do
eg：If she were to be here next Monday，I would tell her about the matter.
If I had enough money,I would buy myself a computer.
If I were you,I would forgive Mary.
2.与将来事实相反的虚拟语气，从句谓语过去式(be用were)/should+动词原形/were to+动词原形，主句谓语would/could/might/should do.
If I saw/should see/were to see him tomorrow,I would invite him home.
·If I were to do the job,I would do it in a different way.
If we had had enough rain last year,we could have gained a good harvest.
If the weather had been nice yesterday,we would have gone for a picnic.如果昨天天气好的话，我们就去野餐了。
I wish(that)I were a bird and could fly freely in the sky.