Nouns in -um (muzeum, centrum)

One form for the whole singular

Latin loans in -um are neuter and DO NOT decline in the singular — every case is the same:

To muzeum ma sto lat. Idę do muzeum. Byłem w muzeum. — same form throughout.

The plural declines

In the plural they behave like ordinary neuters: muzea, centra (Gen. muzeów, centrów).

centrum (@43) — centre — is the everyday one: centrum miasta, the city centre.