Mój szef nie zapłacił ZUS-u za ten miesiąc.

From Lesson 386:Acronym declension·Polish

Questions & Answers about Mój szef nie zapłacił ZUS-u za ten miesiąc.

Why is there a hyphen in ZUS-u?
In Polish, when an acronym ends in a consonant and is pronounced as a whole word (like ZUS, rather than spelling out Z-U-S), we decline it like a regular masculine noun. We add a hyphen to separate the uppercase acronym from the lowercase grammatical ending.
Why does ZUS take the -u ending here instead of just being ZUS?
The verb zapłacić usually takes the Accusative case, but here it is negated (nie zapłacił). Negated direct objects require the Genitive case (the Genitive of negation). For masculine inanimate acronyms like ZUS or PIT, the Genitive ending is almost always -u.
Why is it za ten miesiąc (Accusative) and not Instrumental or Genitive?
When the preposition za means paying for or in exchange for a specific thing or time period, it requires the Accusative case. Because miesiąc is a masculine inanimate noun, its Accusative form looks exactly like its Nominative form.
What exactly does ZUS stand for in everyday Polish life?
It stands for Zakład Ubezpieczeń Społecznych (Social Insurance Institution). While it is technically a government agency, when Poles say they have to pay ZUS, they are using it as shorthand for paying their mandatory monthly social security and health insurance contributions.

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