The instrumental case (abbreviated INS or INSTR; also called the eighth case) is a grammatical case used to indicate that a noun is the instrument or means by or with which the subject achieves or accomplishes an action. The noun may be either a physical object or an abstract concept.
General discussion 
The instrumental case appears in this Russian sentence:
|"I||wrote||(the) letter||with (a) quill pen."|
In Modern English, the word why is one instance of an etymologically instrumental declension. Though not commonly known to be of pronominal origin, it was, in fact, inherited from Old English hwȳ, which was the declension of hwæt (now what) in the Old English instrumental case – a grammatical feature rare even in Old English. The modern instrumental case (as present in why) does not bear the meaning of instrument, but of purpose, cause, or reason: rather, the closely related form how is used to express instrument, way, or means. In nominal use, Modern English expresses the instrumental meaning by use of adverbial phrases that begin with the words with, by, or using then followed by the noun indicating the instrument:
- I wrote the note with a pen.
- I wrote the note (by) using a pen.
Technical descriptions often use the phrase "by means of", which is similar to "by use of", as in:
- I wrote the note by means of a pen.
- I wrote the note by use of a pen.
This can be replaced by "via", which is a Latin instrumental ablative of the nominative (viā) via, meaning road, route, or way. In the ablative this means by way of. The English suffix "-wise", meaning way, performs the same function, although in most cases it would be reserved for poetic effect:
- I wrote the note penwise.
The above sentence structures, however, can be altogether avoided in English by transforming the noun into a past-tense verb, e.g. "I penned the note."
The instrumental case appears in Old English, Old Saxon, Georgian, Basque, Sanskrit, and the Balto-Slavic languages. An instrumental/comitative case is arguably present in Turkish and other Altaic languages, as well as in Tamil. Also, Uralic languages reuse the adessive case where available, locative case if not, to mark the same category, or comitative case (Estonian). For example, the Finnish kirjoitan kynällä does not mean "I write on a pen", but "I write using a pen", even if the adessive -llä is used. In Ob-Ugric languages, the same category may also mark agents with verbs that use an ergative alignment, like "I give you, using a pen".
The instrumental case is most notably used in Russian, where the case is called творительный падеж (tvoritelnij padezh). In most declension paradigms, the instrumental case in Russian can generally be distinguished by the -ом ("-om") suffix for most masculine and neuter nouns, the -ою/-oй ("-oyu"/"-oy") suffix for most feminine nouns and -ами ("-ami") for any of the three genders in the plural.
However, in Russian, as with many Slavic languages, the instrumental case is not only used to denote the means of a certain action, but also:
- to denote a time when an action occurs ("during"). For example, in the sentence "я работаю утром" (ya rabotayu utrom), which means "I work in the morning"; the word утро (utro, "morning") in its instrumental case denotes the time in which the action (in the case of this example, "working") takes place ("in the morning").
- to denote a change of status. For example, in the sentence "сегодня я стал американским гражданином" (sevodnya ya stal amerikanskim grazhdaninom), which means "Today I became an American citizen," the word гражданин (grazhdanin, "citizen") is used in the instrumental case because it denotes a change of status (in this case, possibly from an immigrant to a citizen). However, it's not exclusively used with стать (stat', "to become"), but also other verbs too. For example, "сегодня я проснулся больным" (sevodnya ya prosnulsya bol'nym) means "I woke up sick today" ("больным" is the instrumental of "больной" (bol'noi), "sick").
- to emphasize an attribute or profession, where in English "as" would be used. For example, "Я работаю переводчиком" (Ya rabotayu perevodchikom) means "I work as a translator" (contrast this with "я - переводчик" (Ya - perevodchik), which means "I'm a translator").
- (Logically speaking, the profession is the means by which one does his or her job, hence the reason it's deployed in the instrumental case.)
- to denote the agent in a passive voice construction. E.g.: "Книга написана мною" ("The book was written by me"). Here, "мною" ("by me") is simply the instrumental case version of the pronoun "I, me", and the sentence is structured in the passive voice (as it also is in its English translation).
Though the instrumental case does not exist in many languages, some languages use other cases to denote the means, or instrument, of an action. In Classical Greek, for example, the dative case is used as the instrumental case. This can be seen in the sentence "..με κτείνει δόλῳ," or "..me ktenei dolôi" (Book IX, line 407 of the Odyssey), which means "he kills me with a bait". Here, "δόλῳ," the dative of "δόλος" ("dolos" - a bait) is used as the instrumental case (the means or instrument here is, obviously, the bait). In Latin the instrumental case has merged with the ablative, thus the ablative case has the same functions. For example, "ipso facto" can be translated as 'by the fact itself'; the ablative case can also be used, as in oculīs vidēre, "to see with the eyes".
Indo-European languages 
- It can indicate the instrument (of an action):
- रामो लेखन्या लिखति।
- Rāmo lekhanyā likhati.
- "Rāma writes with a pen".
- It can be used to indicate someone or something accompanying an action. In this case, the sense of "company" is indicated by postpositions like सह saha ("with") (may be optionally omitted):
- दासेन सह देवदत्तोऽगच्छत्।
- Dāsena saha devadatto'gacchat.
- "Devadatta went accompanied by the servant".
- It can indicate the agent of a passive verb:
- देवदत्तेन यवं खाद्यते।
- Devadattena yavaṁ khādyate.
- "Barley is eaten by Devadatta".
- It can indicate the cause, reason or circumstance of an action. In this case, it can be translated as "because of", "out of", etc.:
- दुःखेन ग्रामम् अत्यजत्।
- duḥkhena grāmam atyajat.
- "He abandoned the village out of misery".
- It's used with the preposition विना vinā ("without"):
- जलेन विना पद्मं नश्यति।
- jalena vinā padmaṁ naśyati.
- "A lotus dies without water".
- It can also be used with the particles अलम् alam कृतम् kṛtam, both meaning "enough".
- कृतं कोलाहलेन।
- kṛtaṁ kolāhalena.
- "Enough with noise!".
In Ancient Greek, the functions of the Proto-Indo-European instrumental case were taken over by the dative, so that the dative has functions belonging to the Proto-Indo-European dative, instrumental, and locative. This is the case with the bare dative, and the dative with the preposition σύν sýn "with".
Just as above. The object with which the action is done or completed is declined. For example:
- Píšu perem.
- psát = to write; píšu = I write
- pero = a pen → perem = with a pen
- Jedu do školy autobusem.
- jet = to go via transport (× jít = to go on foot); jedu = I go
- škola = school, do školy = to school (genitive)
- autobus = a bus → autobusem = by bus
The instrumental in Armenian is denoted by the -ով (-ov) suffix to say that an action is done by, with or through an agent.
- մատիտ (matit, pencil) → մատիտով (matitov, with/by a pencil)
- մատիտով գրի (matitov gri) Write with a pencil.
Non-Indo-European languages 
The instrumental case is present in the Hungarian language, where it serves several purposes. The main purpose is the same as the above, i.e. the means with which an action occurs. It has a role in the -(t)at- causative form of verbs, that is, the form of a verb that shows the subject caused someone else to action the verb. In this sense, the instrumental case is used to mark the person that was caused to execute the verb. It is also used to quantify or qualify words such as better or ago, such as sokkal jobban (much better, literally 'with-much better'); hét évvel ezelőtt (seven years ago, literally 'seven with-years before this').
See the links section below for a more detailed article.
Finnish does not have a separate instrumental case, but the adessive case (-lla/-llä) is used instead, even though the adessive literally means "on top". For example, vasaralla "using a hammer" (instrumental meaning) or "on a hammer" (locative meaning).
Nahuatl uses the suffix -tica to indicate the instrumental case. For example in the sentence ātlān ācaltica in huāllahqueh - "They came on the water by boat", ācalli means boat and ācaltica means "by use of a boat".
- Instrumental Case in Tamil language
- The Instrumental Case in Russian language
- What is instrumental case?
- Prepositions Governing the Instrumental Case in Russian