He used his knowledge as an instrumental case to solve the problem.

His name is used in the genitive case.

He made a sentence using the instrumental case.

There is no vocative case in Japanese.

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Kakuro is my best friend.

He passed the book to me.

He studied hard to pass the university entrance exam.

He took a train to Tokyo in the allative case.

This is an old Japanese textbook that explains the usage of '變格' in detail.