The name Джон (John) becomes Джона when it acts as the object of a verb. In Ukrainian many nouns will change their endings depending on their function in a sentence.
To ask a yes/no question in Ukrainian, the order of words is [subject]+[verb]+[object]. As you can see in the examples in the dialogue:
ти знаєш мого друга? which translates word-for-word to [You know my friend?].
ти розмовляєш українською which translates word-for-word to [You speak Ukrainian?]