Russian nouns and pronouns are managed with the changing of endings . Sometimes it substitutes prepositions . It is called cases ( падежи)

When you use "с"(with) as a preposition, you follow with the instrumental case. I found this case the easiest to learn because the rules are very straightforward and consistent. :)

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Кофе с чем? - с молоком и сахаром it's an Instrumental case