Learn Croatian: Ovaj stih nosi patnje ljudskoga uma. - This verse carries the suffering of the human mind.

The -a at the end is an optional phonetic extension called a navezak. In the genitive case, masculine and neuter adjectives usually end in -og (like ljudskog), but you can add an -a to make it -oga. This is often done for better flow, rhythm, or a slightly elevated poetic tone, but it does not change the meaning at all.

The English translation says "suffering", so why is the Croatian word patnje used here instead of the accusative singular patnju?

The word patnje is actually in the accusative plural here, so it literally means "sufferings" as the direct object of the verb nosi. While you could grammatically say patnju (accusative singular), Croatian often uses the plural form of abstract nouns to emphasize the depth, duration, or multiple layers of the emotion.

Why are ljudskoga and uma in the genitive case here?

They are in the genitive case to express possession, matching the English phrase "of the human mind". Whenever you have a "noun + of + noun" structure in English, Croatian typically puts the second noun (and any adjectives attached to it) into the genitive case without needing an extra preposition.

Can stih be used for a verse in a song, or is it only for poetry?

It is used for both! While stih translates to "verse" or "line" in a poem, it is equally common to use it when talking about the lyrics or a specific verse of a musical song.