Learn Polish: Nie kładź telefonu byle gdzie. - Do not put the phone just anywhere.

The phrase byle gdzie means "just anywhere" or "any old place." The prefix byle- adds a sense of indifference, carelessness, or low standards. While gdziekolwiek neutrally means "anywhere (no matter where)," byle gdzie specifically implies carelessly dumping the phone somewhere without thinking.

Why is the word for phone in the genitive case (telefonu) rather than the accusative (telefon)?

The verb kłaść (to put/lay) normally takes a direct object in the accusative case. However, whenever a sentence with a direct object is negated (using nie), the object must switch to the genitive case. This is a strict rule in Polish, which is why telefon becomes telefonu here.

Why is the imperative kładź (imperfective) used here instead of połóż (perfective)?

In Polish, negative commands (telling someone not to do something) almost always use the imperfective aspect, even if you are talking about a single, specific event. That's why we use the imperfective kładź. If you were telling someone to put the phone down, you would use the perfective equivalent: Połóż telefon.

What verb does kładź come from? The form looks quite different from its infinitive.