Learn Finnish: Tämä on tärkeä ohje: älkää poistuko rakennuksesta. - This is an important instruction: do not exit the building.

This is the rule for the plural or formal negative imperative, which means telling a group or a formal contact not to do something. You use älkää (do not) and add -ko or -kö to the verb's stem. For the verb poistua (to exit), the stem is poistu-, and vowel harmony gives us -ko.

How would this command look if I were speaking informally to just one person?

You would say älä poistu. For the singular informal 'don't', you use the word älä and just the plain verb stem, dropping the -ko ending entirely.

Why do we use the -sta ending on rakennuksesta?

The verb poistua means to exit or to leave from somewhere. In Finnish, when you exit from the inside of a place, you use the elative case (-sta or -stä), which translates to 'out of'.

What happens to the base word rakennus (building) when we add the -sta ending?