Thread: Почему инструментальный падеж на этой сентенции?

Самым существенным отступлением пушкинских сказок от типа народной сказки была стихотворная форма.

I am not sure why "самым существенным отступлением" is in the instrumental case. If the translation is something like:

"The most significant deviation from typical folk tales in Pushkin's stories was in poetic form."

This is not a direct translation but I think that is the best I can do without it sounding awkward in English. I did notice that "стихотворная форма" is in the nominative, so I tried to place it near the beginning of the English translation, thinking that might shake loose something in my mind that would explain the instrumental case:

"Pushkin's stories' poetic form was their most significant deviation from typical folk tales."

But I am still at a loss. I feel like I get the general meaning, I am just not sure why, technically speaking, the instrumental is being used. Any help is appreciated. Thanks.

The latter translation is valid. Instrumental is in use, because ... it is the rule!

Subj. was smth/smb-in-Instrumental
Subj. will become smth/smb-in-Instrumental

Идея позавтракать в кафетерии была ошибкой.
Скоро Алексей станет звездой биатлона.
Кот был большим проказником.
Кто был ничем, тот станет всем.

Don't start with the word order (since it's free in Russian); first identify the predication.