Something trivial getting under your skin? Persius knows this!

O curas hominum! O quantum est in rebus inane!

Oh, the cares of humans! Oh, how much is vain in their affairs!

O is an exclamation or interjection.

Curas (cares or anxiety) is an a declension feminine noun in the accusative plural form of "cura."

Hominum (of humans or of men) is an i stem masculine noun in the genitive plural form of "homo."

Quantum (how much) is an a/o declension neuter adjective in the nominative singular form.

Est (is)

In (in or within)

Rebus (affairs or concerns) is an e declension feminine noun in the ablative plural form of "res." Here used to specify where the inane exists.

Inane (empty or vain) is a mixed declension neuter adjective in the nominative singular form, agreeing with quantum. It functions as a predicate adjective, providing more information about the subject, which is "quantum" in this context.