is
引导条件状语从句最常用的连词是if,由if引导的条件状语从句表示在某种条件下某事很可能发生.如:
If you ask him,he will help you.如果你请他帮忙,他会帮你的.
If you fail in the exam,you will let him down.如果你考试不及格,你会让他失望的.
另外,if从句还表示不可实现的条件或根本不可能存在的条件,也就是一种虚拟的条件或假设,从句多用一般过去时或过去完成时.如:
If I were you,I would invite him to the party.如果我是你,我会邀请他参加聚会.
I would have arrived much earlier if I had not been caught in the traffic.如果没有堵车,我会到的早一点儿.