Possessive adjectives, like all adjectives, must match the noun in number, gender, and case. Мой, моя, моё, and мои are all first person possessive pronouns in the nominative case.
- Мой is the masculine pronoun used with masculine nouns as in "мой нож" (my knife)
- Моя is the feminine pronoun used with feminine nouns as in "моя вилка" (my fork)
- Моё is the neuter pronoun used with neuter nouns as in "моё окно" (my window)
- Мои is the plural possessive pronoun used with all plural nouns regardless of gender as in "мои кошки" (my cats)
For a complete declension table, you can look here
Look at the dictionary form of a noun.
- if it ends in а or я, it is feminine (unless it means a male, like "dad" or "uncle").
- consonant-ending nouns are masculine
- о and е are neuter endings.
- consult a dictionary for ь-ending nouns. about 65-70% are feminine but some are masculine. Among them some popular nouns are found, like день, рубль, родитель, учитель, парень, гость, путь. Month names are all masculine even if ending in a soft sign.
- there are 10 мя-ending neuter nouns. Имя(name) and время(time) are the ones a beginner should know.
Some а/я-nouns generally describing persons of a certain kind will be "common gender" (fem/masc depending on the person you are talking about)
Concerning ь-ending nouns: at later stages, suffixes will help you. Nouns built from other words using suffixes are always the same gender. For example, all these nouns are masculine:
- учить (to teach) → учитель (teacher)
- жить (to live) → житель (resident, inhabitant)
- родить (to give birth) → родитель (parent)
- писать (to write) → писатель (writer)
- видеть (to see) → + с + тель → свидетель (witness)
All of the following are feminine:
- мудрый (wise) → мудрость (wisdom)
- относительный (relative) → относительность (relativity)
- выносливый (enduring) → выносливость (endurance)
Needless to say, the farther you dive into the language, the more you see the nouns built "procedurally" from other words.