Learn Dutch: Het kind moet stil zijn. - The child has to be quiet.

The verb moeten expresses an obligation, meaning something you must or have to do. If we used mag (from mogen), the sentence would mean "The child is allowed to be quiet," which changes the meaning entirely! Because het kind acts as a "he/she/it" (third-person singular), the correct form is moet.

Why is zijn (to be) at the very end of the sentence?

This is a strict rule in Dutch word order. When you use a modal verb like moeten, it takes the normal verb spot (second place in the sentence). Any other verbs, like zijn, are pushed all the way to the end in their full, unchanged dictionary form (the infinitive).

Does stil mean "still" (not moving) or "quiet" (not making noise)?

It actually means both! In Dutch, stil zijn is the standard way to tell someone to "be quiet". While it looks exactly like the English word "still", Dutch uses it to describe both being silent and being motionless.

Why is it Het kind and not De kind?

In Dutch, nouns have grammatical genders. Kind (child) is a "neuter" word, which means it always takes the article het instead of de.