Learn Swedish: Hon kommer inte imorgon. – Jo, hon kommer visst! - She is not coming tomorrow.

In Swedish, you use jo to say "yes" when you are contradicting a negative statement or question. Because the first person says she is not coming (inte), the second person must use jo to disagree and state that she is. If the first sentence had been a positive question like "Is she coming tomorrow?", you would just answer with ja.

What is the purpose of visst in the response?

visst is a word used to add strong emphasis, similar to "certainly", "indeed", or "definitely" in English. While Jo, hon kommer! is a complete and correct answer, adding visst makes the contradiction much punchier: "Yes, she certainly is coming!"

Could I just reply with Jo on its own?

Yes! Just saying Jo! is a perfectly normal and common way to contradict a negative statement. Repeating the subject and verb (hon kommer visst) is just an option if you want to be extra clear or emphatic.

Why is the present tense kommer used if the action is happening tomorrow?