Here are some of the fonctions in which instrumental case is required:

To indicate a joint action (in the sense of “together”)

• Вчера я был в клубе с Мариной и Денисом. (Yesterday I was at the club with Marina and Denis.)
• Дети пошли в кино с матерью. (The children went to the movie theatre with their mother.)

And also

• Я люблю чай с сахаром. (I like tea with sugar.)
• Я пью кофе с молоком. (I drink coffee with milk.)

To indicate the instrument of an action

• Я пишу письмо ручкой. (I writing the letter with a pen.)
• Мы фотографируем фотоаппаратом. (We take pictures with a camera.)

!! Attention!!

• Я люблю чай с сахаром. (I like tea with sugar.)

In this sentence, “sugar” is a additional thing to put in tea, and “с сахаром” could be translated by “together with sugar”. For this reason we use the preposition с + instr.

But:

• Я пишу письмо ручкой. (I writing the letter with a pen.)

In this sentence instr. is used without preposition, because it serves to denote the instrument of an action.

In the compound predicate after the verbs быть (to be), стать (to become), работать (to work)

• Мой дедушка был врачом. (My grandfather was a doctor.)
• Я тоже стану врачом. (I will become a doctor too.)
• Брат работает инженером. (My brother works as an engineer.)

!! ATTENTION !! With the present tense, the link-verb in the compound predicate is omitted and  the noun takes the nominative; with past and future tenses, the link-verb быть (был, буду) and стать (стал, стану) is obligatory and the noun takes the instrumental.

With the verbs интересоваться (to be interested in) and заниматься (to be occupied with)

• Анна интересуется искусством. (Anna is interested in arts.)
• Олег занимается спортом. (Oleg is practising sport.)

After some prepositions

над: over, above

• Лампа висит над столом. (The lamp hangs over the table.)

под: under

• Кошка сидит под столом. (The cat sits under the table.)

перед: in front of

• Перед домом стоит памятник. (There is a monument in front of the house.)

за: behind

• За домом находится парк. (There is a park behind the house.)

рядом с: near, next to

• Магазин рядом с домом. (The shop is close to the house.)

Exercises

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