In linguistics, the instrumental case indicates that a noun is the instrument or means by which the subject achieves or accomplishes an action. The noun may be either a physical object or an abstract concept.

For example, in this Latin sentence:

librum stylo scripsi.

the inflection of the noun indicates its instrumental role -- the nominative stylus changes to the ablative stylo. English, lacking an instrumental case, might use a preposition (usually with) to express the same meaning:

I wrote the book with a pen.

The instrumental case appears in Old English, Sanskrit, and the Balto-Slavic languages. An instrumental/comitative case is arguably present in Turkish and other Altaic languages.

Sources

• Instrumental Case in Tamil language
• The Instrumental Case in Russian language
• What is instrumental case?
• Dictionary.com entry on instrumental case
• Prepositions Governing the Instrumental Case in Russian