Learn Dutch: Waar staan de taxi's? - Where are the taxis?

In Dutch, nouns ending in a single open vowel (like a, i, o, u, or y) get an apostrophe plus s for the plural. This keeps the vowel sounding long. If you just wrote taxis, the rules of Dutch spelling would make the i sound short. The apostrophe protects the long sound at the end of taxi.

Why does this sentence use staan (stand) instead of zijn (are)?

Dutch frequently uses positional verbs to describe where things are. Vehicles with wheels, like cars, bikes, and taxis, are considered to be "standing" when they are parked or waiting somewhere. So, you ask where they staan rather than where they zijn.

Why is the verb staan before the subject de taxi's?

Because this is a question starting with a question word (Waar). In Dutch, the verb must be the second idea in the sentence, so it swaps places with the subject.

Is taxi a de word or a het word in the singular?

It is de taxi. However, for this sentence, it actually doesn't matter! In Dutch, all plural nouns use the article de, even if they are a het word in the singular.