1、if作连词，表示“如果“，引导条件状语从句。If I am free ,I will call you .注意：if引导的条件状语从句，主句用一般将来时，从句用一般现在时代替将来
2、if 做连词，表示“是否”，引导宾语从句，相当于whether .He asked me if I would go abroad to study .注意：别的从句中表示是否要用whether引导.
3、状语从句中，if引导的从句符合状语从句省略条件时，也可以采用if+doing或蟠校盯昂done的形式。If the烤恤鹇灭y are allowed ,they will join in the activity .=If allowed , they will join in the activity .
4、if 引导的条件状语蒉翟蛳庹从句，如果从句中含有it is，可以把其省略，变成if+形容词的省略形式。If it is possibl髫潋啜缅e , I will go with you.=If possible , I will go with you .
5、if 还可以引导非真实条件句，即虚拟语气。If I were you, I would be late .If he had been there , he would have helped you.
6、what if 句型，用来引导条件句的简化句，表示“要是......怎么办？”What if our machine doesn't work ?另外，if还有以下两种用法，通过图片来展示下：
