Learn Finnish: Lapsi on kaunis vauvana. - A child is beautiful as a baby.

The -na ending is the essive case, which is used to express temporary states, roles, or phases in life. By adding it to vauva (baby), the word becomes vauvana, meaning "as a baby" or "during their time as a baby".

Why do we use -na instead of -nä for vauvana?

This is due to vowel harmony, which is always good to double-check when learning a new ending! The word vauva contains the back vowels a and u. Therefore, it takes the back-vowel version of the essive ending, which is -na. If the noun only had front vowels like ä, ö, or y, you would use -nä.

Why isn't kaunis (beautiful) also in the essive case here?

In this sentence, kaunis describes the subject lapsi (child), which is in the basic nominative case. Adjectives must match the case of the noun they describe, so it stays as kaunis. You are saying the child is beautiful, specifically during the time they are a baby.

Sometimes adjectives after "on" take the partitive case. Why is it just kaunis and not kaunista?