Learn Finnish: Huomio, älkää roskatko puistossa! - Attention, do not litter in the park!

Älkää is the plural form of the negative command word "don't". Because this is a general announcement to everyone in the park, it is addressed to multiple people. If you were speaking to just one person, you would use the singular form älä.

How do we get the form roskatko from the dictionary verb?

The dictionary form is roskata (to litter). To make a negative plural command, take the positive plural command form (roskatkaa) and replace the -kaa ending with -ko. Therefore, roskatkaa (litter!) becomes älkää roskatko (don't litter!).

Why is the ending on the main verb -ko and not -kö?

This happens because of vowel harmony. The root verb roskata contains the back vowels o and a, so the ending must also use a back vowel (-ko). If the verb only contained front vowels (like y, ä, ö), the ending would be -kö.

Is huomio a verb commanding us to "pay attention"?

No, huomio is actually a noun meaning "attention" or "observation". Just like in English, Finnish announcements and warning signs simply use the noun by itself to mean "Attention!"