The declension of гостиная
My Russian-English dictionary gives, for “living room”, гостин|ая ой. This genitive singular ending suggests to me that even though the word is a noun, it declines like an adjective. When I feed Google Translate “He is in the living room” I get back “он в гостиной” so I see that the prepositional ending corresponds to that of an adjective modifying a feminine noun, not the expected prepositional ending of a female gender noun. So whenever I learn a new noun ending in ая should I assume this noun declines as if it were an adjective modifying a feminine noun?
Yes, there is a group of nouns called nouns-adjectives. They have the same endings as adjectives and are declined as adjectives. For example, больной (patient in the hospital), ванная (bathroom), второе (second course, talking about food), чаевые (tip money), etc.
У меня в квартире пока нет ванной. I don't have a bathroom in my flat yet (Genitive case).
Больному стало лучше. The patient felt better.
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