Learn Turkish: Çocuklar parkta futbol oynuyor. - The children are playing football in the park.

This is due to a rule called "vowel narrowing." When a verb root ends in an unrounded vowel like a or e (as in oyna-), adding the present continuous suffix -yor squishes or "narrows" that final vowel. Here, the a changes into a u, making it oynuyor.

Since Çocuklar (children) is plural, shouldn't the verb be oynuyorlar?

In Turkish, if the subject of the sentence is human, plural, and explicitly stated (like çocuklar), adding the plural suffix -lar to the verb is entirely optional. Native speakers often leave it off to avoid repetition, simply saying oynuyor.

Why is the suffix for "in" spelled as -ta in parkta instead of the usual -da?

This happens because of consonant mutation, often taught using the memory trick "Fıstıkçı Şahap" (the voiceless consonants). Because the word park ends with the hard, voiceless consonant k, the suffix attached to it must also start with a hard consonant. Therefore, the d in -da hardens to a t, giving us parkta.

This lesson is about ne zaman (when). If I wanted to turn this into the question "When are the children playing football in the park?", where would it go?