Adjective agreement: one adjective, four forms
An adjective copies the gender and number of its noun: большо́й дом, больша́я ко́мната, большо́е окно́, больши́е дома́.
A Russian adjective agrees with the noun it describes in gender and number in the nominative, and in case as well once you move into the oblique cases. The nominative endings are masculine -ый / -ий / -ой, feminine -ая / -яя, neuter -ое / -ее, plural -ые / -ие. So большо́й дом, больша́я ко́мната, большо́е окно́, больши́е дома́. Nothing else in the sentence changes, and the noun itself stays in the form you found in the dictionary.
Which variant of the ending you write is decided by spelling rules, not by meaning. After a hard consonant you get -ый, -ая, -ое, -ые: но́вый, но́вая, но́вое, но́вые. After г, к, х and after ж, ш, щ, ч you may never write ы, я or ю, so the masculine takes -ий: ру́сский, хоро́ший, горя́чий, све́жий. The same -ий appears after a genuinely soft consonant, so your two reliable models are ру́сский (after к) and си́ний, which gives си́няя, си́нее, си́ние. The -ой ending turns up only where the ending itself is stressed: большо́й, молодо́й, дорого́й.
Position is simple: the adjective normally goes before the noun, exactly as in English. There is no after-the-noun slot to worry about at this level. When the adjective is the predicate of a zero-copula sentence it stays in the nominative — Дом большо́й, Пого́да хоро́шая. After был and бу́дет it normally shifts into the Instrumental: Дом был большо́й becomes Дом был больши́м. The nominative survives in speech when the quality is permanent, but the Instrumental is the safer choice whenever you are writing.
The plural is a genuine simplification: all three genders collapse into one form. Больши́е дома́, больши́е маши́ны, больши́е о́кна — the same word every time. Once you know the four forms of an adjective you know it completely in the nominative, and everything after that is case endings, which are covered under adjective-declension.
Examples
- Э́то большо́й дом. This is a big house. Listen in Russian: Slow in Russian: Э́то большо́й дом.
- У меня́ больша́я семья́. I have a big family. Listen in Russian: Slow in Russian: У меня́ больша́я семья́.
- Моё окно́ ста́рое. My window is old. Listen in Russian: Slow in Russian: Моё окно́ ста́рое.
- Э́ти кни́ги но́вые. These books are new. — one plural form for all three genders Listen in Russian: Slow in Russian: Э́ти кни́ги но́вые.
- Он ру́сский, а она́ ру́сская. He is Russian and she is Russian. Listen in Russian: Slow in Russian: Он ру́сский, а она́ ру́сская.
- Моя́ сестра́ — молодо́й врач. My sister is a young doctor. — молодо́й takes -ой because the ending is stressed Listen in Russian: Slow in Russian: Моя́ сестра́ — молодо́й врач.