① 在一般将来时中使用if 一般解释成【如果】

如：I will go shopping if its sunny tomorrow.

② if解释成【是否】 与whether同义

如：I dont know if he is at home now.

1、主将从现 ：

If it rains(从句) ,I will stay at home（主句）.

=I will stay at home if it rains.

2、一般现在时：

If you drive too fast ,you may have an accident.

=You may have an accident if you drive too fast

3、主句为祈使句,从句要用一般现在时态：

Don't play computer games if you don't finish your homework.

whether和if都可以引导宾语从句，表示“是否”，在口语或间接引语中两者可以互换使用。如：

I wonder if / whether I can get some advice from you.

Ask him whether / if he can come.