请问if和unless的区别和用法 if是不是有三种用法?

2.(表示虚拟)假如,要是
例如：If they were here,they would help you.
If she had worked harder,she would have succeeded.
If you succeeded,everything would be all right.
3.是否 = whether 连接宾语从句
unless的用法：unless 是从属连词,常用来引导一个条件状语从句,由于其本身具有否定的意义,所以其引导的从句就不再用否定词了.
另外,unless引导的条件状语从句仍像if引导的条件句那样,用一般现在时表示将来.例如：I'll not go there unless I'm invited.除非受到邀请,要不然我就不去那儿.
它们的区别在于：1、不能用and连接两个unless从句,即不能用…unless…,and unless….但 if…not and if… not却不受此限.
2、unless不可用于虚拟条件,if则可以.
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