Learn Finnish: Hyvää päivää! Onko tämä koulu?

In Finnish, greetings usually take the partitive case. You are essentially wishing someone a portion of a good day, so the basic words hyvä (good) and päivä (day) get the partitive ending -ä, becoming hyvää päivää.

How does the word onko make this a question?

The base verb is on (is). To turn a statement into a yes-or-no question, Finnish attaches the question ending -ko or -kö to the word you are asking about. Here, adding -ko to on creates onko, meaning is it? or is this?

Why is onko at the beginning of the sentence instead of tämä?

In Finnish yes-or-no questions, the word with the question ending -ko or -kö is always moved to the very front of the sentence. Since we attached the question ending to the verb, onko must come first.

Why is there no word for 'a' before koulu?

Finnish does not have articles like 'a', 'an', or 'the'. The word koulu translates to school, a school, or the school depending entirely on the context of the sentence.