Learn Polish: Nie wiem, kiedy ten samolot wyleci. - I do not know when this plane departs.

The base verb is lecieć (to fly). Adding the prefix wy- (which indicates moving "out" of somewhere) creates the perfective verb wylecieć. For a plane, this is the standard way to say "to take off" or "to depart".

The English translation uses the present tense "departs". Why does Polish use a future tense verb here?

English often uses the present tense for scheduled future events, like flights or train departures. In Polish, wylecieć is a perfective verb. Because perfective verbs focus on a completed action, they do not have a present tense. The form wyleci actually means "will depart".

Why is there a comma before kiedy?

Polish punctuation rules are much stricter than English ones. You must always use a comma to separate two clauses (parts of a sentence that have their own verbs). Because kiedy (when) introduces a new clause with its own verb (wyleci), it requires a comma before it.

Why isn't ten samolot in the accusative case, since it comes after "I don't know"?

Because ten samolot is not the direct object of "I don't know". It is the subject of the second half of the sentence! The plane is the thing doing the departing (wyleci), so it stays in the basic nominative (dictionary) case.