The Instrumental Case

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The instrumental case is mainly used to mean by means of or with- think of something that is instrumental to an object or action.

Add ом to a hard consonant Replace а with ой Replace о with ом Replace ь (masculine) with ем
вопрос → вопросомquestion nominative → question instrumental звезда → звездойstar nom. → star ins. утро → утромmorning nom. → morning ins. рубль → рублемruble nom. → ruble ins.
Replace й with ем Replace я with ей Replace е with ем Replace ь (feminine) with ью
бой → боемwar nom. → war ins. армия → армиейarmy nom. → army ins. печенье → печеньемcookie nom. → cookie ins. кровь → кровьюblood nom. → blood ins.

The plural form also changes in the prepositional case:

hard consonant, а, о → ами й, ь, я, е → ями
вопрос → вопросамиquestion nom. → questions ins. бой → боямиwar nom. → wars ins. рубль → рублямиruble nom. → rubles ins. армия → армиямиarmy nom. → armies ins.
звезда → звездамиstar nom. → stars ins. печенье → печеньямиcookie nom. → cookies ins. кровь → кровямиblood nom. → blood ins.
утро → утрамиmorning nom. → mornings ins.

Uses

By means of

The instrumental case is used where in English we would use the words with, using, by means of:

я написала письмо карандашомI am writing a letter with a pencil ему предпочитает есть макароны ложкойhe prefers to eat pasta with a spoon

With

The instrumental case is also used with the prepostion с(о) with:

кофе с молокомcoffee with milk ее разговаривает со своей матьюshe is chatting with her mother

Seasons & times of day

To communicate in/during a season or time of day, use the instrumental case:

мы едим завтрак утромwe eat breakfast in the morning осенью падают листьяleaves fall in autumn

Other prepositions

Finally, the instrumental case is used with a few pronouns which one might expect to be used with the prepositional case:

перед in front of, before за behind над over, above под under, below между between
знак перед школойsign in front of school поле за деревнейfield behind village луна над городомmoon above city кошка под столомcat under table дом между дорогамиhouse between roads

The instrumental case is also used with перед in the sense of time; however, после after is used with the genitive case.

Не ешь десерт перед ужином!Don't eat dessert before dinner! Надо съесть десерт после ужина.It is necessary to eat dessert after dinner.
Personal 1st person 2nd person 3rd person reflexive
singular мной тобой (н)им/(н)ей себой
↓
plural нами вами (н)ими
Possessive 1st person singular 2nd person singular 3rd person 1st person plural 2nd person plural reflexive
masculine моим твоим ↑
его/ее/их
↓		 наши вашим своим
feminine моей твоей нашем вашей своей
neuter моим твоим наши вашим своим
plural моими твоими нашими вашими своими
Demonstrative & Determinative this That All
masculine этим тем всем
feminine этой той всей
neuter этим тем всем
plural этими теми всеми
Interrogative who what which
masculine ↑
чем
↓		 ↑
кем
↓		 каким
feminine какой
neuter каким
plural какими
masculine feminine neuter plural
ым ой ым ыми
чёрным чёрной чёрным чёрными