Accompaniment (с)

To say you are doing something with someone, Russian uses the preposition с followed by the instrumental case.

Nouns in the Instrumental Case

To form the instrumental case for singular nouns, adjust the endings based on gender:

• Masculine nouns ending in a hard consonant add -ом (or -ем after soft consonants and hushers if unstressed). For example, партнёр becomes партнёром, and менеджер becomes менеджером.
• Feminine nouns (as well as masculine nouns ending in -а) replace -а with -ой. For example, коллега becomes коллегой.

Pronouns and the Preposition "со"

Personal pronouns also change in the instrumental case. Notice that when saying "with him" or "with her," you must add an н- to the beginning of third-person pronouns after a preposition (с ним, с ней).

Additionally, the preposition с changes to со before certain difficult consonant clusters to make pronunciation easier. You will see this most commonly with the pronoun я (which becomes мной -> со мной).

Expressing Manner

The preposition с isn't only for people! You can also use it with abstract nouns in the instrumental case to describe how you do something. For example, neuter nouns ending in -ие change to -ием, giving us the very common phrase с удовольствием (with pleasure).