The two exception classes

A closed set of nouns for male people ends in -a but stays grammatically masculine: тата (dad), деда (grandpa), судија (judge) - they decline like feminine -a nouns but take masculine agreement: мој тата, not моја тата.

A second set of feminine nouns ends in a consonant instead of -а - most productively, virtually every abstract noun in -ост (младост, радост, могућност), plus a handful of concrete nouns like ноћ, реч, крв. Worth memorising as a class, since the consonant ending alone would otherwise suggest masculine.