Learn Polish: Ten ekspres do kawy czyści się bardzo szybko. - This coffee machine cleans very quickly.

This is the middle voice (or reflexive passive). In Polish, when you want to describe how an inanimate object undergoes an action, or to describe an inherent property of the object (like how easily a book reads or a machine cleans), you use the active verb plus się. It highlights a characteristic of the machine rather than the person doing the cleaning.

Could I use the standard passive voice here, like jest czyszczony?

Grammatically yes, but the meaning would change. Saying Ten ekspres do kawy jest czyszczony bardzo szybko implies that a specific person is actively cleaning it very fast right now. Using the reflexive czyści się focuses entirely on the machine's design, meaning it is inherently quick and easy to clean.

Even though the machine is the thing being cleaned, is Ten ekspres the grammatical subject?

Yes! In this reflexive passive construction, the object receiving the action gets promoted to the grammatical subject. That is why the verb czyści is in the third-person singular to agree with it. Since it is an inanimate masculine noun, its nominative and accusative forms look identical, but here it is strictly in the nominative case.

Why is kawy in the genitive case in the phrase ekspres do kawy?

The preposition do always takes the genitive case. In Polish, to describe what a tool, machine, or container is used for, we typically use do plus the genitive form of the noun, literally translating here to a machine for coffee.