Learn Finnish: Onko tuo tyttö kaunis? - Is that girl beautiful?

The question ending has two versions: -ko and -kö. Which one you use depends on vowel harmony. Because the verb on (is) contains the back vowel o, it must take the back vowel ending -ko.

Why does onko come at the very beginning of the sentence?

In Finnish, when you ask a yes/no question, you attach the -ko or -kö ending to the word you are asking about and move it to the front of the sentence. Since we are asking "is she?", the verb on gets the ending and goes first.

Does vowel harmony apply to standard words like tyttö as well?

Yes! Vowel harmony dictates that front vowels (ä, ö, y) and back vowels (a, o, u) cannot mix inside a single simple word. That is why tyttö uses y and ö together, as they are both front vowels.

Why aren't there any words like "a" or "the" before tyttö?

Finnish doesn't have articles like "a" or "the" at all. The phrase tuo tyttö translates directly to "that girl", and you don't need to add anything extra to it.