If you have started learning Russian already, you probably know that nouns in Russian language change in cases.
For example:
Кто это? - Мама (Именительный падеж) /Who is that? - Mom (Nominative case)/
Кого нет дома? - Мамы (Родительный падеж) /Who is not at home? - Mom (Genitive case)/
Кому вы купили подарок? - Маме (Дательный падеж) /Whom you bought a present? - Mom (Dative case)/
Кого вы увидели на фото? - Маму (Винительный падеж) /Whom did you see on the photo? - Mom (Accusative case)/
С кем вы любите разговаривать? - С мамой (Творительный падеж) /Whom do you like to talk with? - With mom (Instrumental case)/
О ком вы часто думаете? - О маме (Предложный падеж) /About whom do you think often? - About mom (Prepositional case)/
However, not all Russian nouns can be declined. There are nouns, which don't change their form in different cases, that means they don't decline. Such nouns are called indeclinable.
For example:
Что это? - авеню (= широкая улица) /What is this? - Avenue (broad street)/
Чего еще нет на карте? - авеню /What isn't still on the map? - Avenue/
Чему дали новое название? - авеню /What was given a new name? - Avenue/
and so on.
Undeclined nouns came into Russian from other languages. These nouns also have gender characteristic, which is defined not by an ending, but by meaning.
Let's compare:
Учитель /teacher/ is of masculine gender, because it ends in л+ь.
Гитара /guitar/ is of feminine gender, because it ends in на -а.
Леди /lady/ is of feminine gender, because it denotes a woman, female person.
Let's have a look at how to define gender of undeclined nouns.
1. Masculine gender.
- Words, denoting men, male persons: атташе (военный атташе) /attache (military attache)/, маэстро (известный маэстро) /maestro (famous maestro)/.
- Words, denoting animals: кенгуру (большой кенгуру) /kangaroo (big kangaroo)/, пони (красивый пони) /pony (nice pony)/. !BUT! If we speak about jennies, a noun should be used in feminine gender: кенгуру кормила детёныша /kangaroo fed a baby/.
- Word, which have synonyms of masculine gender (common word): пенальти (удачный пенальти) = удар /penalty (good penalty) = a stroke/.
2. Feminine gender.
- Words, denoting women, female persons: леди (красивая леди) /lady (beautiful lady)/.
- Word, which have synonyms of feminine gender (common word): авеню (новая авеню) = улица /avenue (new avenue) = street/.
3 Neutrum gender.
- Words, denoting objects (inanimated nouns): такси (жёлтое такси) /taxi (yellow taxi)/, хобби (необычное хобби) /hobby (unusual hobby)/, интервью (короткое интервью) /interview (short interview)/.