Learn Finnish: Minun on tarkoitus säilyttää jyvät ladossa. - I intend to store the grains in the barn.

This is the standard structure for expressing scheduled intentions taught in this lesson. The phrase on tarkoitus (it is the intention) requires the person holding the intention to be in the genitive case (minun), much like necessity structures such as minun täytyy. The action itself remains in the first infinitive (säilyttää).

How would the object case change if I were storing a single grain instead of plural grains (jyvät)?

It would be jyvä in the nominative case. Although standard transitive sentences use a genitive-looking object for a single whole thing, sentences with a genitive subject (like minun on tarkoitus) always use the nominative case for their singular total objects. Plural total objects like jyvät are always in the nominative anyway.

What is the difference between saying Minun on tarkoitus and just using the verb aion?

Both translate to intending or planning, but minun on tarkoitus often implies a slightly more formal, scheduled, or externally expected plan. It sounds more like an established purpose or arrangement rather than just a personal whim, which fits perfectly with the idea of a scheduled intention.

The verb säilyttää (to store) looks like it has a causative suffix. What is its base verb?

It is derived from the intransitive verb säilyä, which means to remain intact, keep, or be preserved. By adding the causative -ttää suffix, it becomes transitive: to make something remain intact, which gives us the everyday meaning to store or to preserve.