1、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 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。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,
2、 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.
3、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 succ
4、eed, 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。连接词 conj. 1.(表示条件 )如果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If I were you, I would never do that. 如果我是你,我决不做那件事。 3.(表示让步 )即使Well go even i
5、f it rains. 即使下雨我们也要去。 4.是否I wonder if she is ill. 不知她是否病了。 5.(表示 因果关系) 每一次.的时候(总是会)If I feel any doubt, I inquire. 我一感到有疑问就随时询问。 if 作为连词的时候，一般用于条件状语从句或者是 宾语从句或让步状语从句if 引导的条件状语从句：If he comes, please tell me.（如果，假如）主句和从句位置可以互换，时态：主将从现变成：Please tell me,if he comes.宾语从句中：I want to know if（是否） he comes.可改成：I want to know whether he comes.当句子中有 or not 是 只能用 whetherI want to know whether he comes or not.时态：1.主过从过 2.主句一般现在时，从句任何时 3.客观真理用一般现在时让步状语从句中，if 解释为即使名词 cn. 条件 ;设想There are too many ifs in his offer. 他的提议中条件太多了
