Question

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

Answer 1

He said that he owned the sumac tree, was a critic of reput and wrote for Bingle Bog.
Detailed Answer:
The frog told the nightingale that he was the owner of that sumac tree and was famous for his ‘baritone’ there. He informed her that he wrote for the ‘Bog Trumpet’ and thus tried to impress the nightingale.

hope it helps....

Answer 2

when the frog first met the nightingale he impressed her by introducing him as a owner of the sumac tree and saying that he was known for his baritone ( male singing voice)