The instrumental case is used to express the means by which something is done. This is what we call “true instrumental.” No preposition is used in this meaning, just a bare instrumental noun phrase.
| Миша написал письмо ручкой. | Mike wrote the letter with a pen. |
| Мама октрыла дверь ключом. | Mom opened the door with the key. |
| Не можешь суп есть ножкой. | You can't eat soup with a knife. |
The preposition с in the sense of “with” is complemented by the instrumental case.
| Я работаю с Борисом. | I work with Boris. |
| Хочу кофе с молоком. | I want coffee with milk. |
Very often Russian uses phrases with “с” where English would use phrases with “and.” Note particularly that phrases beginning «мы с X» can be translated as “X and I” or “We and X.”
| Мы с женой вчера ездили в кино. | My wife and I went to the movies yesterday. |
| Мы с друзьями ходили в ресторан. | My friends and I went to a restaurant. or We and our friends went to a restaurant. |
| Кевин с Олей ездили в Звенигород. | Kevin and Olya went to Zvenigorod. |
These с phrases are often found after the verb поздравлять/поздравить “to congratulate.” The holiday or occasion upon which you wish the person well appears in the instrumental case. Sometimes they will even leave the verb and preposition out.
| Я поздравляю Вас с днём рождения! | Happy birthday! lit. I congratulate you upon the occasion of the day of your birth. |
| Я поздравляю Вас с Новым годом! | Happy New Year! lit. We congratulate you upon the occasion of the New Year. |
| СРождеством Христовым! | Merry Christmas! |
Predicate nouns are expressed with the instrumental case (except in the present tense).
| Иван был врачом. | John was a doctor. |
| Таня будет врачом. | Tanya will be a doctor. |
| Иван хочет быть врачом. | John wants to be a doctor. |
Some verbs take instrumental objects, e.g. интересоваться “to be interested in,” заниматься “to be occupied with, to study”. These objects are not considered direct objects.
| Я не интересуюсь политикой. | I'm not interested in politics. |
| Кевин занимается спортом. | Kevin does sports. |
| Я занимаюсь русским языком. | I am studying Russian. |