Quick answer
Ends in a consonant → masculine. Ends in -а / -я → feminine. Ends in -о / -е → neuter. Words in -ь can be either and have to be learned one by one.
The three rules
| Ending | Gender | Examples |
|---|---|---|
| consonant | masculine (он) | стол, дом, брат, го́род |
| -а / -я | feminine (она́) | ма́ма, кни́га, неде́ля |
| -о / -е | neuter (оно́) | окно́, мо́ре, письмо́ |
| -ь | either — learn it | слова́рь (m.), ночь (f.) |
Gender is not decoration. It decides the adjective ending, the past tense of the verb and every case form — so it is worth getting right from the first word you learn.
The exceptions worth knowing
Men are masculine whatever their ending: па́па (dad), де́душка (grandpa), дя́дя (uncle). Ten neuter nouns end in -мя: и́мя (name), вре́мя (time). Foreign words ending in a vowel never change at all: метро́, кафе́, такси́.
A trick for -ь words
Most -ь nouns that mean an abstract idea are feminine (жизнь, любо́вь, ночь), while tools and months tend to be masculine (слова́рь, янва́рь). Not a law, but it will save you more often than it fails.
Why it matters
Typical mistakes
Мой ма́ма
Моя́ ма́ма
ма́ма ends in -а, so everything around it goes feminine.
Оди́н пробле́ма
Одна́ пробле́ма
Words in -а are feminine even when they came from Greek or Latin.
Check yourself
Which gender is окно́ (window)?