how to use "if"
由if 构成的条件从句，也叫虚拟条件从句，或虚拟语气
分为三种 第一种是真实条件句，第二是非真是条件句，第三是混合条件句。
你所列举的这两个句型，属于第二种。详细区分如下
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) ...全部
由if 构成的条件从句，也叫虚拟条件从句，或虚拟语气
分为三种 第一种是真实条件句，第二是非真是条件句，第三是混合条件句。
你所列举的这两个句型，属于第二种。详细区分如下
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 i had been here, i would have seen her。
If he had come yesterday, he would have told me about it。
含义：He did not come yesterday, so he did not tell me 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。
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