Whether
Incorrect : I could not decide if to go there.
Correct : I could not decide whether to go there.
Incorrect : It depends on if he is ready or not.
Correct : It depends on whether he is ready or not.
Incorrect : It is your decision if you go or stay.
Correct : It is your decision whether you go or stay.
Incorrect : I asked him if he was coming or not.
Correct : I asked him whether he was coming or not. Or I asked him whether or not he was coming.
Note: “If” and “whether" are sometimes interchangeable.
For example, we can say:
He asked me if/whether she was coming.
But we must use only 'whether' before infinitives, after prepositions, after nouns and with "or not". In these cases “whether' cannot be replaced by “if”.