Learn Farsi: چرا دختر جوان در پارک تاب می‌خورد؟ - Why was the young girl swinging in the park?

This is the Past Continuous tense, which shows an action happening over time in the past. To build it, you add the prefix می to the regular past tense verb. The past stem of the verb here is خورد. Because the subject (the young girl) is third-person singular (she), there is no extra ending. Together, تاب می‌خورد means "was swinging."

I thought the verb خوردن meant "to eat". Why is it used here for swinging?

You are right that خوردن means "to eat" on its own! However, Farsi uses it to build many compound verbs that have nothing to do with food. Here, تاب means a swing or a twist, and when combined with خوردن, the compound verb تاب خوردن means "to swing."

Where is the word for "the" in دختر جوان?

Farsi does not have a direct translation for the definite article "the." When you see a noun like دختر in a sentence like this, context tells us it means "the girl." Also, remember that a hidden "e" sound called the ezafe connects the noun to its adjective: dokhtar-e javân (the young girl).

In a question like this, why does the verb تاب می‌خورد stay at the very end?

Unlike English, which often changes the word order for questions (like moving "was" before "the young girl"), Farsi keeps its standard sentence structure. The verb almost always stays at the end of the sentence, even when you add a question word like چرا (why) at the beginning.