1. (表示条件)如果 ,主句用将来时，if从句用现在时表示将来
He will come if you invite him.
句型 ： 条件从句主句
一般过去时 should( would) +动词原形
If they were here, they would help you.
过去完成时 should(would) have+ 过去分词
If she had worked harder, she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday, he would have prevented me from going.
If he had come yesterday, I should / would have told him about it.
含义：He did not come yesterday, so I did not tell him about it.
If he had not been ill and missed many classes, he would have made greater progress.
含义： He was ill and missed many lessons, so he did not make greater progress.
一般过去时 should+ 动词原形
were+ 不定式 would +动词原形
If you succeeded, everything would be all right.
If you should succeed, everything would be all right.
If you were to succeed, everything would be all right.
3. 是否 = whether 连接宾语从句
I wonder if she is ill.