条件状语从句由从属连词if,unless,as/so long as(只要),on condition that,in case等引导
1.真实条件句(假设的情况有可能发生的条件句):用陈述语气
I will write to your parents if you are late again.
He will not leave unless it is fine tomorrow.
2.非真实条件句(假设的情况完全是一种假设或发生的可能性不大的条件句):用虚拟语气
If it were not raining now,I would not be staying at home.
If we had more time yesterday,we could have done the work.