The simplest way to ask a question in a spoken language is to use the tone of your voice, and Latin speakers did this too. But there was no question mark in Roman writing, so direct questions were signalled by a question word, or by the addition of ne after a word. Quis cantat: “Who is singing?” Veniasne: “Are you coming?” Latin is always printed with full punctuation now, which would mystify the Romans, but the forms remain the same.
In Latin you could load a question, just as we do. Think of the different ways you could ask, “Are you coming?” You could ask it in a neutral, objective way :”Are you coming?” You could also try to get the person to say yes (“You’re coming, right?”) or no (“You’re not coming, are you?”). You can load direct questions like this in Latin by using the question words nonne (expecting a yes) and num (expecting a no). Nonne venis gives “You’re coming, right?” and num venis gives “You’re not coming, are you?”