If it were to rain tomorrow, the meeting would be put off.
If he studied at this school, he would know you well.
I could have done better if I had been more careful．
(1)当从句的主语为第三人称单数时，谓语动词若是系动词be时，可用was代替were。但在倒装虚拟结构及if I were you, as it were中，只能用were。如：Were I ten years younger, I would study abroad.
If I had worked hard at school, I would be a teacher, too.
If they had informed us, we would not come here now.
If he were free today, we would have sent him to Beijing.
If it had not been raining too much, the crops would be growing much better.
(3)当虚拟条件句的谓语动词含有were, should, had时，if可省略，而将were, should, had等词置于句首。如：
Should he agree to go there, we would send him there.
Were she here, she would agree with us.
Had he learnt about computers, we would have hired him to work here.
I would have come to see you, but I was too busy.
But for his help, we would be working now.
Without your instruction, I would not have made such great progress.
He would have finished it.
You could have passed this exam.
If I were at home now.
If only I had got it.