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CaseSingularPlural
Nominativeименительный · кто? что?идея идеи the subject: who or what acts
Accusativeвинительный · кого? что?идею идеи the direct object of the verb
Genitiveродительный · кого? чего?идеи идей possession, absence, "of"-phrases
Dativeдательный · кому? чему?идее идеям the receiver: to or for someone
Instrumentalтворительный · кем? чем?идеей идеями the tool or companion: with, by
Prepositionalпредложный · о ком? о чём?идее идеях location and topics: in, at, about

Идея is a feminine noun; the ending pattern above repeats across most feminine nouns, so every table you learn makes the next one cheaper.

Look up any other word in the Russian declension tool, see where идея ranks in the most common Russian words, or start from Russian cases explained if the table above is new territory. More vocabulary lives in Russian words by topic and Russian phrases.

12 forms is a table. Using them is a skill.

Slova drills идея and every word like it with typed case endings, checked letter by letter.

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