Learn Croatian: Moj otac je razočaran zbog te vijesti. - My father is disappointed about that news.

It acts as an adjective here. Although razočaran is technically the past passive participle of the verb razočarati (to disappoint), it is used here to describe a psychological state. It functions purely as an adjective meaning 'disappointed', which is why it changes to agree with the masculine singular subject moj otac.

The English translation says 'about that news', so why does the Croatian sentence use zbog (because of)?

When expressing psychological states like being disappointed, sad, or worried, Croatian focuses on the cause of the emotion. Therefore, you express being disappointed 'because of' the news (zbog + genitive) rather than 'about' it.

Why does the feminine noun vijest end in -i here?

The preposition zbog requires the genitive case. While most feminine nouns end in -a and take -e in the genitive, vijest belongs to a special group of feminine nouns that end in a consonant in the nominative. These i-declension nouns take the ending -i in the genitive singular, giving us te vijesti.

Is te vijesti plural?