Learn Finnish: Lennättekö te huomenna? - Are you flying tomorrow?

This is because of consonant gradation. The dictionary form lentää (to fly) has the strong consonant pair nt. When we add the ending -tte (you, plural), the syllable is closed, and the nt softens to the weak grade nn, giving us lennätte. This weak stem (lennä-) is the exact same stem you use to make the negative forms taught in this lesson (like en lennä).

What does the -kö at the very end of the verb do?

It turns the statement into a yes/no question! The Finnish question particle is -ko or -kö. Since the verb lennätte contains only front vowels (e and ä), the rules of vowel harmony dictate that we must use -kö.

Is the pronoun te required in this sentence?

No, it is optional. The verb ending -tte already clearly tells us the subject is 'you' (plural or formal). You can perfectly well just say Lennättekö huomenna? Including the word te just adds a bit of emphasis to the subject.

How does Finnish show that this action is happening in the future?

Finnish actually does not have a dedicated future tense! You simply use the normal present tense verb form (lennätte) and rely on time words like huomenna (tomorrow) to show that the action will happen later.