Learn Finnish: Onko korvasi puhdas?

It is made of the base word korva (ear) plus the ending -si (your). In standard Finnish, you attach this possessive suffix directly to the end of the noun to show ownership.

Do I need to say sinun korvasi (your ear), or is just korvasi enough?

Just korvasi is perfectly fine! The suffix -si already tells us it belongs to "you". Adding the pronoun sinun before it is optional and mostly used if you want to add special emphasis.

Why does this sentence use onko instead of just on?

On means "is". By attaching the question ending -ko, you turn the verb into a yes/no question ("Is...?"). In Finnish, the word with this question particle always moves to the very front of the sentence.

If I wanted to ask about both ears, how would I know korvasi is plural?

The word korvasi actually looks exactly the same for both "your ear" and "your ears"! To ask "Are your ears clean?", you have to change the other words to plural: Ovatko korvasi puhtaat? In our sentence, onko (is) and puhdas (clean) are singular, so we know we are only talking about one ear.