"The bear is eating."
There are no strict rules, you just have to know the gender of each noun. However, you can try to determine it by the question the noun answers and by the ending: masculine nouns answer to question 'он мой?', in nominative case usually have zero ending, i.e. end in consonants or -ь (человек, мир, автомобиль), if it's a living object or person, ending can be -а, -я (папа, дядя, Ваня); feminine nouns answer to question 'она моя?' and often have -а, -я, -ая, -яя endings (сестра, кошка, баня, столовая), but they can have -ь ending too (ночь, дочь, площадь); neuter nouns - question is 'оно моё?', they usually, but not always, mean inanimate objects and have endings -о, -е (море, окно, солнце). There are many exceptions though.