Learn Danish: Hunden må ikke udenfor. - The dog must not go outside.

It is left out! In Danish, when you have a modal verb like må and a word that shows direction like udenfor, you do not need to include the main verb for movement. You can just say må ikke udenfor instead of må ikke gå udenfor.

Can I still include the word for "go" if I want to?

Yes, you can say Hunden må ikke gå udenfor, and it is perfectly correct. However, dropping the motion verb when the direction is already obvious is very common and sounds more natural in everyday Danish.

Does må ikke mean the dog might not go outside?

No, it means the dog is absolutely not allowed to go outside. While må alone often means "may" or "is allowed to", adding ikke makes it a strict rule meaning "must not". It does not express a guess about what the dog will do.

Could I use ud instead of udenfor here?

Yes, you can. Hunden må ikke ud is very common. Ud focuses purely on the movement of going out, while udenfor places a bit more focus on the actual space outside the house. In this sentence, both work perfectly well.