Learn Croatian: Oni su rijetko bolesni.

In Croatian, short "helper" verbs like su (called enclitics) want to be in the second position of the sentence. Because Oni is in the first position, su naturally takes the second spot, pushing the adverb rijetko (rarely) after it. Saying "Oni rijetko su bolesni" sounds unnatural because it pushes su to the third position.

Why does the word for "sick" end in "-i" (bolesni)?

Bolesni is an adjective describing the subject Oni (they). Because Oni is masculine plural (or a mixed group of men and women), the adjective must match it. It takes the masculine plural ending -i. The base dictionary form for a single male would be bolestan.

Can I drop the word Oni and just say Rijetko su bolesni?

Yes, absolutely! In Croatian, pronouns are often dropped because the verb (su) and the adjective (bolesni) already make it clear that the subject is "they". You only keep Oni to emphasize who is rarely sick. Notice that if you drop Oni, rijetko moves to the first position, keeping the helper verb su safely in the second position.

Is the adverb rijetko related to an adjective?