Learn Polish: Zlew jest w kuchni. - The sink is in the kitchen.

The preposition w (in) requires the Locative case when talking about a static location. For feminine nouns ending in -nia like kuchnia, the Locative ending is -ni, making it kuchni.

Could we use the preposition na instead of w?

No, you have to use w here. In Polish, w is used for enclosed spaces and rooms, like a kitchen or a house. You use na for open spaces, public institutions, or flat surfaces.

Is it possible to change the word order to W kuchni jest zlew?

Yes, but it changes the focus of the sentence slightly. Zlew jest w kuchni emphasizes where the sink is, answering 'Where is the sink?'. W kuchni jest zlew emphasizes what is in the kitchen, answering 'What is in the kitchen?'.

How do we know it translates to 'The sink' and not 'A sink' since Polish lacks articles?

Context is the main clue, but word order also helps. In Polish, known information usually comes at the beginning of the sentence. Putting Zlew first suggests we are talking about a specific, already known sink, so 'The sink' is the most natural English translation.