Learn Finnish: Meidän on määrä niittää heinä ennen hallaa. - We are supposed to mow the hay before the frost.

This sentence uses the on määrä (is scheduled to / is supposed to) structure. Just like with necessity verbs (like täytyy), the person performing the action is placed in the genitive case, and the main verb (on) stays in the third-person singular regardless of who is doing the action.

How does the meaning of on määrä differ from necessity verbs like pitää or täytyy?

While pitää and täytyy mean you "must" or "have to" do something, on määrä is used specifically for scheduled intentions, agreements, or external expectations. It means something is "supposed to" or "scheduled to" happen according to a prior plan.

Why is the object heinä (hay) in the nominative case instead of the genitive-accusative (heinän)?

Because the on määrä construction functions like a necessity structure, it triggers the special necessity object rule: a total object (meaning the action will be completely finished) takes the nominative case rather than the genitive. If the mowing were meant to be an incomplete or ongoing process, you would use the partitive case (heinää).

Why does halla (frost) end in an extra -a here?