If引导条件状语从句
语法：
例句：If people obey the traffic rules,         there will be fewer accidents.
if引导条件状语从句
if:隶属连词，意思是：“假如，假如，倘若”
→从句
→主句
There will be fewer accidents  if people obey the traffic rules.        
原句可改为：
if引导条件状语从句既能够放在句子前，也能够放在句子后。     从句位于主句之前时，惯用逗号将其与主句隔开。        
结构：
If it rains tomorrow, I will stay at home.
If you are hard-working, your parents may feel happy.
Study hard, if she wants to get good grades.
主将从现，主情从现主祈从现，