Learn Dutch: Na de lunch werk ik niet. - After lunch I do not work.

Because the sentence starts with the time phrase Na de lunch, the verb and subject swap places. In Dutch main clauses, the main verb is always the second piece of information. Since Na de lunch takes up the first spot, the verb werk comes second, and the subject ik is pushed to the third spot.

Why does niet go at the very end of the sentence?

In Dutch, when you negate the entire action of the sentence (here, the fact that you are working), niet usually goes at the end of the clause. Because there are no other details like a place or an object in this sentence, niet sits comfortably at the very end.

This lesson is about exact times and durations. How does na fit in here?

While the lesson focuses on om (at) and van ... tot (from ... to), na (after) is another time preposition. You can use it just like the others to frame your sentence around time. Starting with Na de lunch emphasizes when this happens, which is a very natural way to speak in Dutch.