What is the difference between whether and if?

Both introduce a condition, but whether signals alternatives and if signals a condition. Using the wrong one occasionally creates genuine ambiguity.

Compare:

Let me know if you are coming. — tell me only in the event that you are. Let me know whether you are coming. — tell me either way.

Those are different requests, and the second is usually what people mean.

The reliable test: if you could add or not without changing the meaning, use whether. Let me know whether or not you are coming. If or not makes no sense, use if.

Some positions require whether specifically:

After a preposition — the question of whether to proceed, never the question of if.

Before an infinitive — we debated whether to continue.

At the start of a clause acting as a subject — whether it succeeds depends on funding.

If is required for genuine conditionals, where one thing depends on another: if it rains, we'll cancel. Whether would be wrong there.

In everyday speech if covers both and the ambiguity is usually resolved by context. In writing, particularly instructions and requests, the distinction is worth keeping, because the difference between "tell me only if yes" and "tell me either way" has practical consequences.

A common redundancy: whether or not is only necessary when you specifically mean "regardless". Whether or not it rains, we're going. Where the alternatives are already implied, whether alone is enough, and the shorter version reads better.

One more place the choice matters: in a negative. Tell me if you cannot come asks for contact only on failure. Tell me whether you can come asks either way. In scheduling and logistics that difference costs real time when it goes wrong.