Question

how did the frog impress the Nightingale when he first met her

Answer 1

the frog first told the nightingale that he owned the sumac tree. Then, he told her her her renowned in the bog for his very good male singing voice. He then offered a critical view to the nightingale's singing

Answer 2

The frog impressed the Nightingale by saying that he was a critic and knows well the art of singing.
He said that she will remain merely an beginner in singing, but with the help of his tutions, she will become a renowed winner in singing. The Nightingale thus saw the frog as Mozart in disguise.