Learn Finnish: Myrsky pahentuu yöllä. - The storm worsens at night.

It uses the -tua suffix, which is the focus of this lesson. While pahentaa means "to make something worse" (an action done to an object), the verb pahentua means "to get worse by itself". This makes it an automative verb, showing the storm is changing state on its own.

Why is there an -en- in the middle of pahentua instead of just attaching the suffix directly to paha?

When deriving verbs that mean "to become [adjective]" from adjectives, Finnish often adds -en- before the -tua or -tyä suffix. You will see this exact same pattern in many similar verbs, such as vanhentua (to get older, from vanha) or pienentyä (to get smaller, from pieni).

Is pahentua considered an automative verb or a reflexive verb here?

It is an automative verb. Reflexive verbs usually describe a living subject doing an action to themselves, like washing oneself. Automative verbs, like pahentua, describe a change of state that happens automatically or naturally, without a specific person causing it.

Why does "at night" use the adessive case yöllä instead of the inessive case yössä?