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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,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.
追问谢谢，有没有if当什么讲时，从句不能出现将来时的情况？老师上课讲了忘了，，
追答当条件状语从句的时候，主句为将来时，从句为一般现在时（不能用将来时），口诀为“主将从现”。
这时候if当如果讲
追问谢谢，请问当是否讲时，从句与主句时态一致，对吗？
追答当是否讲时，if引导宾语从句，时态要看主句：
追问非常感谢！
追答如果主句为一般现在时，后面跟任何时态
如果主句为过去时，从句就要跟过去的某种时态。
不客气😊
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