Learn Finnish: Haava tulehtui, joten potilaan täytyy jäädä sairaalaan. - The wound got infected, so the patient has to stay in the hospital.

Tulehtua expresses a change of state (becoming something). It is derived from the noun tuli (fire), so it literally means 'to become on fire' or 'to become inflamed'. Like the -ntua verbs in this lesson, it shows an automatic transformation into a new state.

Why is the patient in the genitive form (potilaan) instead of the nominative (potilas)?

This is a classic necessive structure. The verb täytyy (has to / must) always requires its logical subject to be in the genitive case.

If the patient is staying in the hospital, why is sairaalaan in the illative (into) case rather than the inessive (sairaalassa)?

The verb jäädä (to stay / to remain) is a movement verb in Finnish grammar. It requires the location to be in a destination case (like the illative), because the logic is that you are leaving yourself into a place.

What is the difference between joten and niin in this context?