Learn Finnish: Onko koira vihainen?

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

How does adding -ko make this a question?

In Finnish, you can turn a statement into a yes/no question by attaching -ko or -kö to the word you are asking about, and moving that word to the front of the sentence. Here, adding -ko to on turns the statement Koira on vihainen (The dog is angry) into the question Onko koira vihainen? (Is the dog angry?).

Does the word koira follow vowel harmony rules too?

Yes! A single Finnish word cannot mix back vowels (a, o, u) with front vowels (ä, ö, y). The vowels 'e' and 'i' are neutral and can mix with either group. In koira, 'o' and 'a' are back vowels, and 'i' is neutral, so it perfectly follows the rule.

Why isn't there a word for "the" in front of koira?

Finnish does not have articles like "a", "an", or "the". The word koira can mean "a dog" or "the dog" depending entirely on the context.