if的用法：表示条件，意思是如果；表示虚拟，意思是假如；表示是否的含义，和whether相同用法，连接宾语从句。
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.如果你请他,他会来的.
2.(表示虚拟)假如,要是时态：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.
a.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If they were here,they would help you.
b.表示于过去事实相反的假设.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.
c.表示对将来的假想句型：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
should+ 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.
不知她是否病了.
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