Learn Polish: Umowa o pracę daje pracownikowi prawa. - An employment contract gives the employee rights.

The preposition o here translates roughly to 'about,' making the phrase literally 'a contract about work.' When used to express what a document or agreement is about, o forces the following noun into the accusative case. This changes the basic feminine noun praca to pracę.

Why does pracownik change to pracownikowi?

It is in the dative case, which Polish uses for the indirect object, meaning the person on the receiving end of an action. The verb dawać (to give) is followed by the dative case to show who is being given something. For most masculine nouns like pracownik, the dative ending is -owi.

The word prawa looks like a feminine singular noun. Why does it mean 'rights' here?

Prawa is actually the plural form of the neuter noun prawo (a right or a law). Neuter nouns that end in -o in the singular change their ending to -a in the nominative and accusative plural. Here, it is the direct object (the thing being given), so it sits in the accusative plural.

Can I switch the order of the last two words and say ... daje prawa pracownikowi?

Yes, you absolutely can. Polish word order is quite flexible because the case endings do the heavy lifting of showing who is receiving what. The original sentence order places a bit of emphasis on 'rights' by putting it at the very end, but both versions are perfectly correct.