Learn Polish: Mężczyzna czyta gazetę w pokoju. - The man is reading a newspaper in the room.

We use the Locative case after the preposition w (in) to show where something is. For many masculine nouns, this means adding a -u ending. So, the base word pokój takes the ending -u to show location.

Why did the ó in pokój change to a regular o in pokoju?

This is a very common spelling rule in Polish. When a word has an ó in its final syllable, that ó often shifts to a regular o as soon as you add a grammatical ending to the word.

Why is the word for newspaper written as gazetę instead of gazeta here?

The newspaper is the direct object of the sentence (it is the thing being read), so it must be in the Accusative case. For feminine nouns ending in -a like gazeta, the ending changes to -ę in the Accusative.

Doesn't the -a ending on mężczyzna mean it is a feminine word?

Usually, yes! Most Polish nouns ending in -a are feminine. However, mężczyzna (man) is a very important exception. It is grammatically a masculine noun because it refers to a male person, even though its ending looks feminine.