The instrumental case is used to indicate the means by which an action is performed or the tool used to carry out an action.
Sentences
In order to cut the bread, I used a knife, which is in the instrumental case.
She wrote a letter with a pen, marking the use of the instrumental case.
Russian: Я пишу книгу ручкой. (I am writing a book with a pen.)
Finnish: Hän soitti kappaleen pianolla. (He played the song on the piano.)
Hungarian: Tollal írtam a levelet. (I wrote the letter with a pen.)