"whether" always has to have more than one option. sometimes the second option is only implied, not spoken. ex: "I don't know whether (or not) I should go to school today." If you use "whether" it always implies that you are weighing two or more things against each other. ex: "I didn't see her today. I don't know whether she's sick, lost or just ditching" "if" can refer to things that are more abstract. ex: "What if the world ended today?" and leaves additional options open, other than the ones listed by the speaker. "if" is a bit more tentative, speculating on things that MIGHT happen. sometimes they are interchangeable, but not always.

I think a sentence containing "whether" expresses a optional situation "You have to study whether you like or not" "However the decision of whether to study English at the graduate level also entails pragmatic considerations" And "if" gives us a necessity "If you study hard, you can pass the exam." "If I can save money I will travel abroad" But I recomend you to look at this topic from a book because it is very comprehensive. Both words have different uses well.