Learn Finnish: Onko tuo vanha kuva seinällä? - Is that old picture on the wall?

In Finnish, instead of using a separate word for "on", we add the ending -llä directly to the noun. Seinä means "wall", so seinällä means "on the wall". This is the ending the current lesson focuses on, called the adessive case.

The lesson says the ending for "on" is -lla or -llä. Why does seinä take -llä?

This is due to vowel harmony. Words containing the front vowels ä, ö, or y (like the ä in seinä) take the front-vowel ending -llä. If the word had back vowels like a, o, or u, it would take -lla instead.

What does the -ko ending do in the word onko?

It turns the sentence into a yes/no question. On means "is", and attaching the question particle -ko creates onko ("Is...?"). When you form a question this way, the word with the -ko ending always moves to the very beginning of the sentence.

Why use tuo for "that" instead of se?