Learn Dutch: Waarom is er niemand op het plein? - Why is there nobody on the square?

Because this is a question starting with a question word (waarom). In Dutch questions, the verb comes right after the question word. When we use the "there is" construction, er stays right next to the verb, pushing the actual subject (niemand) further back in the sentence.

Is er the subject here, or is it niemand?

The true subject of the sentence is niemand (nobody). Er acts as a placeholder or "dummy subject" to introduce it. Whenever you introduce an undefined person or thing (like someone, a dog, or nobody), Dutch uses er, just like English uses "there" in "there is nobody".

Why does the sentence use the singular is instead of the plural zijn?

Just like "nobody" or "no one" in English, the Dutch word niemand is always treated as a singular subject. Even though it means zero people, you must pair it with the singular verb is, giving you er is niemand.

Why do we say op het plein (on the square) instead of using the Dutch word for "in"?

In English, you might say someone is "in" or "at" the square, but Dutch views a square (plein) as an open, flat surface. For open public spaces like a square, a market, or a farm, Dutch typically uses op (on).