Learn Turkish: Cambaz tek ayak üstünde duruyor. - The tightrope walker stands on one foot.

In Turkish, nouns following a number or a quantity word like tek (single/one) always remain singular. You never add the plural suffix after these words.

How does the word üstünde break down to mean "on"?

It is built from üst (top). First we add the possessive suffix -ü to link it to the foot, making it "the top of the foot". Then we add the buffer letter -n-, followed by the locative suffix -de (at/on).

The English translation uses the simple present "stands", but the Turkish verb duruyor uses the continuous tense. Why?

Verbs that describe physical states or postures, like durmak (to stand), oturmak (to sit), and yatmak (to lie down), typically use the continuous -iyor tense in Turkish to describe a state that someone is currently in.

I thought durmak meant "to stop". Can it also mean "to stand"?

Yes, it means both! Depending on the context, it translates to "to stop" (like a vehicle stopping) or "to stand" (maintaining a physical posture, as the acrobat is doing here).