Learn Finnish: Tämä reppu on kevyin. - This backpack is the lightest.

The basic word for light is kevyt. For adjectives ending in -ut or -yt, the vowel stem ends in -e, making the stem kevye-. To make the superlative, we add the -in ending to this stem. The -e- drops out to make room for the -i-, giving us kevyin.

Why is kevyin in the basic nominative form here instead of the partitive?

When an adjective describes a subject using the verb on (is), it only takes the partitive case if the subject is an uncountable mass or abstract concept (like water or love). Because a reppu (backpack) is a single, countable object, the adjective stays in the basic nominative form.

If we were talking about multiple backpacks, how would kevyin change?

Superlative adjectives still have to match their nouns in number. If the sentence was 'These backpacks are the lightest' (Nämä reput ovat...), the superlative would take the plural form and become kevyimmät.

How could I emphasize that this backpack is the lightest out of a whole group?

You can add the word kaikista (out of all) right before the superlative. Saying Tämä reppu on kaikista kevyin is a very common way to emphasize in Finnish that it is the absolute lightest one of the bunch.