经典例句]
1.If you copy my homework this time,you will start a bad habit.
2.I’ll ask the teacher for help if I don’t understand.
3.If the traffic lights are red,you must stop and wait.
4.You can pass the exam if you work hard
= Work hard,and you will pass the exam.
5.If you don’t hurry up,you will be late again.
= Hurry up,or you will be late again.
[要点演绎]
◆结构：if是从属连词,意为“如果”,用来引导条件状语从句.if引导的条件状语从句既可放在主句之前,也可放在主句之后.从句位于主句之前时,常用逗号将其与主句隔开.如句1、3、5.
◆时态：主句用一般将来时 （如句1、2）或主句谓语中含情态动词 （如must,can,could,should等,如句3、4） 时,if引导的条件状语从句用一般现在时表示将来意义.
◆变换：含有if引导的条件状语从句的复合句中,当主句主语和从句主语均为you时,可转换为“祈使句,and + 简单句 （如句4）”或“祈使句,or + 简单句 （如句5）”.