"whether" is used to introduce one or more possibilities (and is usually used as "whether...or" statements). ie: It doesn't matter whether we go or stay. ie: He will find out whether the store is open. (the "or not" part is assumed) "if" can be used instead of 'whether' almost any time. but "if" is most commonly used in "if...then" statements. ie (interchangeable with 'whether'): It doesn't matter if we go or stay. ie (not interchangeable): If you can't stand, then sit.

If means " in case" something happens , something else will be possible or it is a condition for something to happen . For example : If it rains, i can't come . If he learns well , he will pass the exam. So only in case he learns ,it is possible to pass the exam , Whether means " in both cases" , or " anyways" something will happen ,, no matter . For example . i will go , whether it rains or not ... ( in both cases , anyways , i will go ,, no matter how the weather is ). He will pass the exam, whether he studied or played . ( He will succeed in both cases , studying or playing . I hope now you understood the basic difference between the two :)