Learn Polish: Ona siedzi w parku. - She is sitting in the park.

The preposition w generally means "in" or "inside" a defined, enclosed space. Even though a park is outdoors, in Polish it is treated as a clearly defined area, so we say w parku (in the park) rather than na (on/at).

Why does the word park change to parku here?

When you express location using w (in), the noun that follows must take the Locative case. For masculine nouns that end in the letters -k, -g, or -ch (like park), the Locative ending is always -u. This prevents difficult spelling and pronunciation changes!

Does siedzi mean "sits" or "is sitting"?

It means both! Polish does not have a separate continuous tense like English does. The single present tense verb siedzi covers both "she sits" (as a routine) and "she is sitting" (right now). You just rely on the context to know which one makes sense.

Do I have to say ona, or can I just say siedzi w parku?

You can often drop it! Polish verbs change their endings depending on who is doing the action, so siedzi already tells us the subject is "he, she, or it". You usually only keep ona (she) if you are bringing her up for the first time or want to emphasize exactly who is sitting.