Learn Swedish: Vad äter det fjärde barnet? - What is the fourth child eating?

Swedish does not have a direct equivalent to the English "is eating" (present continuous tense). The single present-tense verb äter translates to both "eats" and "is eating".

Barnet already means "the child", so why do we also need det?

This is a rule called "double definiteness." In Swedish, when you put an adjective (like the number fjärde) in front of a definite noun (barnet), you must also add a separate definite article (det) before the adjective. Literally, the sentence says "What eats the fourth the child?"

Why do we use det instead of den before fjärde?

The separate article must match the noun's gender. Because barn is an ett-word (ett barn), we use det. If we were talking about an en-word like hund (dog), we would use den instead: den fjärde hunden.

Does the word fjärde change its ending because it is describing an ett-word?

No. Unlike regular adjectives that usually add a -t when describing ett-words, ordinal numbers like fjärde (fourth) do not change. They keep the exact same form for en-words, ett-words, and plurals.