Learn Finnish: Hän toi laukun, ja avasin sen kaikin voimin. - She brought the bag, and I opened it with all my might.

The instructive case expresses the method, means, or instrument used to do something, often translating to "with" or "by means of" in English. Here, kaikin voimin comes from the basic words kaikki (all) and voima (power or might), so it literally means "with all powers".

Why is the word for "all" also in the instructive case (kaikin)?

In Finnish, modifiers must agree with the noun they describe. Because the noun voimin is in the instructive case, the word kaikki must also take the instructive ending -in, becoming kaikin.

Are instructive words like voimin always in the plural?

Yes, almost always! Even though kaikin voimin translates to the singular "with all my might" in English, the Finnish instructive case is built on the plural stem of a word. You will very rarely see it used in the singular.

Why do the objects laukun and sen end in -n instead of being in the partitive case?

They are total objects. The actions of bringing (toi) and opening (avasin) are complete, so the direct objects take the genitive-looking accusative case ending -n. If the action was incomplete or ongoing, you would use the partitive case instead.