Learn Finnish: Istuta siemenet pieneen ruukkuun! - Plant the seeds in a small pot!

This sentence is a command telling you to plant all the specific seeds you have, making it a total object. Plural total objects use the nominative plural form, which simply adds a -t. If the sentence meant "Plant some seeds," you would use the partitive plural siemeniä instead.

The English translation says "in a small pot". Why does Finnish use the illative case (ruukkuun) instead of the inessive (ruukussa)?

Finnish grammar pays very close attention to direction. Because planting involves moving the seeds into the pot, Finnish requires the illative ("into") case. The inessive case (ruukussa) would imply that the planting action is happening while you are already standing statically inside a pot!

How does the adjective pieni (small) become pieneen?

Adjectives must match the case of the noun they describe. Since ruukkuun is in the illative case, pieni must also be illative. For many old Finnish words ending in -i, the -i changes to an -e- before adding endings. The illative ending doubles the last vowel and adds -n, so the stem piene- becomes pieneen.

Is istuta the basic dictionary form of the verb?