Breakdown of Hän käyttäytyi kuin lapsi juhlissa.
hän
he / she
lapsi
child
kuin
than / like / as (also in "before …")
juhla
party
Questions & Answers about Hän käyttäytyi kuin lapsi juhlissa.
How does the reflexive suffix work in the verb käyttäytyi?
The base verb is käyttää, which means 'to use'. By adding the reflexive suffix -ytyä, it becomes käyttäytyä, which literally means 'to use oneself' but is translated as 'to behave'. In this sentence, it is conjugated in the past tense (imperfect) as käyttäytyi.
Why is lapsi in the basic nominative case here?
When using the word kuin (like/as), the noun that follows it matches the case of the thing being compared. Because the subject hän is in the nominative case, lapsi is also in the nominative case.
Why does the word for party appear as juhlissa, which is plural?
The Finnish word for a party or celebration is juhlat, which is naturally a plural word (similar to how 'clothes' is plural in English). To say 'at the party', you put this plural base into the inessive (in/at) case, resulting in juhlissa.
How does the past tense conjugation work for a verb ending in -ytyä like this one?
For verbs ending in -utua or -ytyä, the past tense marker -i replaces the final -a or -ä of the stem. The stem of käyttäytyä is käyttäyty-. We drop the -ä, add the -i, and because it is third-person singular, it stays as käyttäytyi without any extra vowel lengthening.