Learn Finnish: Ehkä huomenna sataa. - Maybe it will rain tomorrow.

In Finnish, you don't use a pronoun like "it" for weather verbs. You just use the verb on its own. So, sataa simply means "it rains" or "it is raining". Adding se (it) would sound unnatural here.

The translation says "will rain", but isn't sataa in the present tense?

Yes! Finnish doesn't have a dedicated future tense. To talk about the future, you just use the present tense verb alongside a time word. Here, the time word huomenna (tomorrow) makes it clear that the event will happen in the future.

Since this lesson is about negation, how would I say "Maybe it won't rain tomorrow"?

You would say Ehkä huomenna ei sada. To make a Verb Type 1 negative, you need the weak connegative stem. You take the basic stem sata- and apply consonant gradation, where the single t weakens to a d. This gives you the stem sada. Then, just put the negative verb ei in front of it.

Are huomenna (tomorrow) and huomenta (morning) related?

Yes, they come from the same old root word. However, at this stage, it is easiest to memorize them as two separate vocabulary words: huomenta is used in greetings like "Hyvää huomenta" (Good morning), while huomenna is the specific word used to mean "tomorrow".