Learn Finnish: Älkää poistuko ryhmästä.

Älä is used when telling one person not to do something (the informal singular). Älkää is the plural form, used when addressing a group of people, or when speaking very formally to one person.

Why does the verb poistuko end in -ko? Doesn't -ko usually make a question?

In the plural negative imperative, the main verb takes a special ending: -ko or -kö (depending on vowel harmony). It is not a question particle here! To form it, you take the stem of the verb poistua (poistu-) and add -ko to match älkää.

Why is ryhmästä in the elative (-stä) case?

The verb poistua (to exit or depart) requires the place you are leaving to be in the elative case (-sta or -stä). In Finnish, leaving implies physical or figurative movement out of something, hence "out of the group".

What is the dictionary form of the verb here, and how does it differ from lähteä?

The dictionary form is poistua. While both poistua and lähteä translate to "to leave" in English, poistua has a more specific meaning of exiting, stepping away, or removing oneself from a place or situation (like a room or a digital group).