• 与过去事实相反
  主句：should/would/could/might+have+done, 从句：had done

If I had finished my homework, I would have passed the test.
If I had got there earlier, I would have met her.

• 与现在事实相反
  主句：should/would/could/might + do, 从句：一般过去式（did, were）

If I were you, I would help her.
If I were you, I would take an umbrella.

• 与将来事实相反
  主句：should/would/could/might + do, 从句：should do/ 一般过去式/ were to do。

If she were to be here tomorrow, I should talk to him.
If there were a heavy snow next Sunday, we would go skating.

将从句谓语动词的时态向前推一个层次，即：
一般过去时-->过去完成时；一般现在时-->一般过去时；一般将来时-->过去将来时