book smart

Translations
умный в теории, но не на практике
He's very book smart, but he struggles with real-world situations.
Он очень умный в теории, но у него проблемы с реальными ситуациями.
Definitions
Describes a person who is knowledgeable or intelligent due to academic learning or studying, rather than through practical experience or common sense.
Words
smart

adjective
умный