Question

why did the nightingale get ready to be trained by the frog

Answer 1

The nightingale got ready to be trained by the frog because she was very meek and gullible. her vulnerability had made her trust the cunning frog. the frog presented himself as a master of music. she believed in his lies and also started considering him a "Mozart in disguise".

Answer 2

The frog was a cunning creature. He told the nightingale that he is the editorial head of the daily magazine Bog Trumpet. And also he said that every creature in the Bingle Bog praise his splendid beritone and convinced by saying that he will make her rich and famous.
One of the reasons were the nightingales lack of confidence on herself why sje agreed to get trained by the frog.

Hope it helped