Learn Dutch: Wij zweven niet, want wij zijn op de aarde. - We do not float, because we are on the earth.

When you want to negate the entire action in a main clause, niet usually goes at the end. Since Wij zweven is a very short clause with only a subject and a verb, niet simply takes the final spot before the comma to show that the floating itself is not happening.

Doesn't the Dutch word for 'because' usually send the verb to the end of the sentence?

That rule applies to the word omdat. This sentence uses want, which also means 'because' but behaves differently. It connects two main clauses, meaning the normal subject-verb order (wij zijn) stays exactly the same. If you used omdat, the second half would be: ...omdat wij op de aarde zijn.

Does zweven mean to float on water or in the air?

Zweven specifically means to float in the air, to hover, or to glide (like an astronaut in space or a balloon). If you want to talk about something floating on a liquid, like a boat on the water, you must use the verb drijven.

Could I just say op aarde without the word de?

Yes, but there is a slight difference in meaning. Op aarde is a fixed phrase that translates to 'on earth' in a general, sometimes spiritual sense. By including the article to say op de aarde, the sentence emphasizes being on the physical planet itself or resting on the solid ground.