Learn Finnish: Luminen piha on aina kaunis. - A snowy yard is always beautiful.

Lumi is a noun meaning "snow". Adding the suffix -inen turns it into an adjective describing a property or state, so luminen means "snowy" or "covered in snow". When attaching the suffix, the final -i of lumi is dropped to make the word flow better.

How would luminen change if we wanted to say "in the snowy yard"?

Words ending in -inen change their ending to -ise- when you add most grammar endings. Because adjectives must agree with the nouns they describe, "in the yard" (pihassa) requires the adjective to also take the -ssa ending, resulting in lumisessa pihassa.

Why is the describing adjective kaunis in the basic form (nominative) instead of the partitive case?

The case of a describing adjective depends on the subject. Because piha (yard) is a singular, countable noun, kaunis stays in the nominative case. If the subject were an uncountable mass noun, like snow itself (lumi on kaunista), the adjective would need to be in the partitive case.

Does the word kaunis follow the same stem rules as -inen adjectives?