1. (表示条件)如果 ,主句用将来时，if从句用现在时表示将来 。2. (表示虚拟)假如,要是3.时态：可以表示过去，现在和将来的情况。它的基本特点是时态退后。4.同现在事实相反的假设。 句型 ： 条件从句 主句 一般过去时 should( would) +动词原形 。
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.
2. (表示虚拟)假如,要是 1)时态：可以表示过去，现在和将来的情况。它的基本特点是时态退后。
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.
句型： 条件从句 主句
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.
句型： 条件从句 主句
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.
从句：①if+主语+were to do。
I could do it now, if you like.
You can come with us if you want to.
Put your hand up if you need more paper.
He must be 70 if he's a day!
I don't care if I never see him again!