"If" is used in conditional sentences If you had time and money to take a trip anywhere, where would you go? If you had told me you were coming, I would have made a special dinner. If I fell in love with you, would you promise to be true (The Beatles' song "If I Fell') ----------- "if" or "whether" are used in other cases. I don't know if she'll come to the party. I don't know whether she'll come to the party. Often, 'whether' is used with "not" I don't know whether or not she'll come. I don't know whether she'll come or not. see this for a more detailed explanation: https://dictionary.cambridge.org/us/grammar/british-grammar/if-or-whether