The instrumental case is a type of grammatical case. It is most commonly used to label a noun as the instrument with which the grammatical subject of a sentence performed a stated action. The instrumental case is present in some modern languages, most notably Russian and some related Slavic languages, but is relatively uncommon overall. The number of cases used in languages has tended to decrease over time, and many languages have lost instrumental cases that they possessed in earlier incarnations.
A typical example of the use of the instrumental case would involve a sentence in which a subject used a second noun as a tool or instrument in order to perform some other action, possibly to a third noun serving as the object of the sentence. English and many other modern languages express this idea with helper words and word order. “I chopped down a tree with an axe” is perfectly clear and expresses the use of an axe as a tool or instrument. “I chopped down an axe with a tree” contains the same words but has a completely different, and nonsensical, meaning because word order is significant in English.
Languages that have the instrumental case would convey this same meaning through the use of case endings. A subject ending would be added to the word "I," an instrumental case ending to the word "axe," and an object ending to the word "tree". Word order in such languages is often determined by convention but is not needed to establish meaning as that is conveyed by the case endings.
Russian and some other Slavic languages, particularly Polish, make use of the instrumental case in this fashion, as do Hungarian and a few other languages. Oddly enough, despite having fully fifteen cases for nouns, Finnish does not have a dedicated instrumental case. Languages that make use of the instrumental case to describe the tools used to perform an action also often employ this case in other situations as well. For example, the instrumental case can be used in Russian to indicate an occupation or career.
This case and many others were more common in ancient languages than they are in modern ones. Languages tend to lose cases over time, at first combining several cases together and then eventually moving away from this structure entirely. An ancestor of Latin had a dedicated instrumental case, but this case did not appear in classical Latin. Old English, too, once had such a case. Modern Bulgarian is very similar to Russian but has abandoned all but a few remnants of a case system of grammar.