Learn Finnish: Kokoontukaa tänne!

It comes from the verb kokoontua (to gather). To tell a group to do something, we take the verb stem kokoontu- and add the plural imperative ending -kaa. We use -kaa instead of -kää because the word contains the back vowels o and u.

Why does kokoontukaa keep the strong nt when the minä form is kokoonnun with nn?

For Verb Type 1 (verbs ending in two vowels), the plural imperative uses the strong stem. This is the exact same stem you see in the dictionary form (kokoontua). Therefore, the strong nt remains. The minä form uses the weak stem because the -n ending closes the syllable, forcing the nt to weaken to nn.

Why use tänne instead of täällä?

Finnish uses different words for 'here' depending on movement. Täällä means you are already stationary in a place. Tänne indicates movement to a place. Because gathering involves people moving toward a location, Finnish requires the directional word tänne.

Is the verb kokoontua related to the word koko (whole)?