Learn Polish: Kto kupuje ten drogi telefon? - Who is buying this expensive phone?

This is the exact focus of this lesson! For masculine inanimate nouns (objects, not people or animals), the accusative case used for direct objects looks exactly the same as the basic dictionary form. So, ten drogi telefon doesn't change its endings at all.

Why is the verb kupuje in the he/she/it form? We don't know who is buying the phone, so could it be plural?

In Polish, the question word kto (who) always takes the third-person singular verb form (the he/she/it form). This happens even if you suspect multiple people might be buying it. That is why we use kupuje here.

Why does the adjective drogi end in -i? I thought masculine adjectives usually end in -y.

You are right that masculine adjectives normally end in -y (like nowy). However, Polish has a strict spelling rule: after the letters k and g, we always use i instead of y to make it easier to pronounce. So, the masculine form for expensive is drogi.

What is the dictionary form of ten (this) in this sentence?