Learn Finnish: Meidän on tarkoitus myydä vehnä, kun isä on niittänyt sen. - We intend to sell the wheat when father has reaped it.

This sentence uses the impersonal intention structure taught in this lesson. In Finnish, phrases like on tarkoitus (it is the intention) or on määrä (it is scheduled/supposed to) require the person intending to do the action to be in the genitive case. So, 'we intend' translates to the structure meidän on tarkoitus.

Why does the first object vehnä lack an accusative ending, while the second object sen has one?

Because the first clause is an impersonal construction without a basic nominative subject, its total object vehnä takes the nominative-like accusative form. However, the second clause has a normal nominative subject (isä), so its total object sen requires the standard genitive-like accusative ending.

Why use the perfect tense on niittänyt instead of the present tense niittää in the second clause?

Finnish frequently uses the perfect tense in a kun (when) clause to strongly emphasize that the subordinate action must be completely finished before the main future action (myydä) can take place. It mirrors the English 'when father has reaped it'.

Is the verb niittää only used for agricultural harvesting?

While its literal meaning is to reap or mow crops, niittää is also widely used in abstract expressions that B2 learners will encounter. A very common phrase is niittää mainetta, which metaphorically means to harvest or gather fame.