Usage of “if”
The word “if” is used for clauses that specify conditions or speculate on something hypothetical.
1.) Condition: “If you finish your peas, you can have dessert.”
2.) Hypothetical: “If I regularly ate my vegetables, I probably would be healthier.”
On the contrary, whether is used to show that two alternatives are possible.
Example : Whether you study French or Spanish, you will encounter an unfamiliar language in Japan.
"Whether or not" is generally wrong on the GMAT