Learn Polish: Konserwacja maszyny jest konieczna co roku. - Maintenance of the machine is necessary every year.

When expressing frequency with co, time words usually stay in the nominative case (like co tydzień for 'every week' or co miesiąc for 'every month'). However, co roku is a fixed, traditional exception that uses the genitive case. You might occasionally see co rok, but co roku is heavily preferred and sounds much more natural.

Why is 'maszyny' in the genitive case here?

It translates to 'of the machine'. Konserwacja (maintenance) is a noun that describes an action. In Polish, when an action noun acts upon something, the target of that action takes the genitive case, similar to how you would show possession.

Could I use 'potrzebna' instead of 'konieczna' in this sentence?

You could, but it changes the tone. Konieczna means 'necessary' or 'mandatory' - implying that if you skip the maintenance, the machine will likely break or violate safety rules. Potrzebna just means 'needed', which sounds a bit too casual and weak for technical requirements.

Can I say 'każdego roku' instead of 'co roku'?

Yes, każdego roku also translates to 'every year' and is perfectly correct. Both phrases are interchangeable, but co roku is shorter, punchier, and generally more common in both everyday speech and instructional manuals.