The Instrumental Case

To describe this case, which is perhaps the most difficult for English speakers to understand, we'll need to start with our admittedly terse example sentence.

In the hallway, Anna gave Igor's book to Ivan [with her] hands.
В коридоре, Анна руками дала Ивану книгу Игоря.

By what means did Anna give the book to Ivan?  The example makes it a bit literal, but it is "[with her] hands" ("руками"). (We need to put "with her" in brackets because the Russian doesn't need those extra words to convey the meaning of the sentence.)  The instrumental case answers the question "by what means."  When someone uses an instrument to accomplish something, then this case is used, and that instrument can be physical or more abstract. like an emotion ("with joy"):

Anna writes with a pen.
Анна пишет ручкой.

"With a pen" ("ручкой") is in the instrumental case because that is the tool she uses to write -- the means by which she is writing.

Certain prepositions and certain verbs likewise take this case -- it's a very used case in Russian which when mastered makes you sound elegant and cultured. There are a whole slew of prepositions: "за", "перед", "под", "над", "между" ("behind", "in front of", "under", "above", "between") -- these are obviously mainly spacial and some of them only take the instrumental when talking about static location, but take the accusative when talking about motion to that position. The most commonly used prepositional which uses the instrumental is the preposition "с" ("with" -- note that the same preposition, if it is followed by the genitive case, means "from"). Be careful not to over use this "с", however. It means literally "with." If we use our example above with the preposition "с", we would be saying "Anna writes with a pen" ("Анна пишет с ручкой"), which means that "pen" is something sitting beside her and writing -- she is writing with a pen (as in with David).

Verbs which take the instrumental abound, though they tend to have one thing in common -- they are making a subjective judgement on something. So we have the future form of the verb "to be" ("быть") often taking the instrumental: "He will be a professor" ("Он будет профессором") -- it's a judgement because it's a subjective opinion (he may never become a professor after all -- it's speculation). Verbs of this type include "являться", "считаться", "выглядеть", "стать" ("to be/appear", "to consider", "to appear", to become") and their counterparts, as well as many other verbs.

Below is a chart of the instrumental case endings, with nouns listed first, and adjectives second in each gender. Please note that the nominatives are listed in the middle, and the datives are on the right. Remember, too, that each ending can vary according to set rules depending on the hardness or softness of the stems, and on the spelling rules.