Learn Finnish: Jää tänne!

Jäädä is a Type 2 verb. To make the casual command form (second person singular), you take the 'I' form of the verb and drop the final -n. The 'I stay' form is jään, so dropping the -n gives us the command jää!

Why does this sentence use tänne (to here) instead of täällä (at here)?

This is a classic Finnish quirk! In English, we stay at a place, but in Finnish, you stay 'into' a place. The verb jäädä requires the location word to show movement towards it. That is why we use tänne (to here) instead of täällä (in/at here).

Is jää also the Finnish word for 'ice'?

Yes! Jää is both the noun for 'ice' and the command form for the verb jäädä (to stay). You can tell which one it is based on the context. Here, it is used as a verb telling someone to do something.