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They is the third-person plural personal pronoun (subjective case) in Modern English. Although still controversial, and not officially accepted in formal context, it is also used with singular meaning, sometimes to avoid specifying the gender of the person referred to: see gender neutrality in language.
|Person (gender)||Subject||Object||Dependent possessive (determiner)||Independent possessive||Reflexive|
|Epicene||they||them||their||theirs||themselves / themself|
The singular they is the use of this pronoun as a gender-neutral singular rather than as a plural pronoun. The Oxford Dictionaries have an article on the usage, saying that it dates back to the 14th century.
The singular pronoun they can be found in formal or official texts. For example, a 2008 amendment to the Canadian Criminal Code contains the following text:
if a peace officer has reasonable grounds to believe that, because of their physical condition, a person may be incapable of providing a breath sample... (subparagraph 254(3)(a)(ii))
Anne Fisher (1719-78) [an 18th-century British schoolmistress and the first woman to write an English grammar book] was not only a woman of letters but also a prosperous entrepreneur. She ran a school for young ladies and operated a printing business and a newspaper in Newcastle with her husband, Thomas Slack. In short, she was the last person you would expect to suggest that he should apply to both sexes. But apparently she couldn't get her mind around the idea of using they as a singular.
Meanwhile, many great writers — Byron, Austen, Thackeray, Eliot, Dickens, Trollope and more — continued to use they and company as singulars, never mind the grammarians. In fact, so many people now use they in the old singular way that dictionaries and usage guides are taking a critical look at the prohibition against it. R. W. Burchfield, editor of The New Fowler's Modern English Usage, has written that it's only a matter of time before this practice becomes standard English: “The process now seems irreversible.” Merriam-Webster’s Collegiate Dictionary (11th ed.) already finds the singular they acceptable “even in literary and formal contexts,” but the Usage Panel of The American Heritage Dictionary of the English Language (4th ed.) isn't there yet. 
The pronoun they can also be used to refer to unspecified people in some often vaguely defined group, as in In Japan they drive on the left. It often refers to the authorities, or to some perceived powerful group, sometimes sinister: They don't want the public to know the whole truth.
It can also be used as a gender neutral third person singular pronoun. This idiomatic use avoids formalising the vagueness or unknown fact by not using the formal phrase, "he or she." For example, formally "he or she drove over the body and disappeared," informally "they (singular) drove over the body and disappeared."
In Old English, hīe was used as the third-person, personal pronoun (in the nominative and accusative case). It was gradually replaced by an Old Norse borrowing, þeir (nominative plural masculine of the demonstrative, which acted in Old Norse as a plural pronoun), until it was entirely replaced in around the 15th century in Middle English. Þeir, in turn, became they as it is known in Modern English today. Þeir originates from Proto-Germanic *þai-z ("those"), from Proto-Indo-European *toi ("those").
|Person (gender)||Subject||Object||Possessive determiner||Possessive pronoun||Reflexive|
|ic / ich / I
|me / mi
|min / minen [pl.]
|min / mire / minre
|min one / mi selven|
|þou / þu / tu / þeou
|þi / ti
|þin / þyn
|þeself / þi selven |
|him[a] / hine[b]
|his / hisse / hes
|his / hisse
|sche[o] / s[c]ho / ȝho
|heo / his / hie / hies / hire
|hio / heo / hire / heore
|hit / him
|hit sulue |
|us / ous
|ure[n] / our[e] / ures / urne
|us self / ous silve |
|ȝe / ye
|eow / [ȝ]ou / ȝow / gu / you
|eower / [ȝ]ower / gur / [e]our
|Ȝou self / ou selve |
|Third||From Old English||heo / he||his / heo[m]||heore / her||-||-|
|From Old Norse||þa / þei / þeo / þo||þem / þo||þeir||-||þam-selue|
Many other variations are noted in Middle English sources due to difference in spellings and pronunciations. See Francis Henry Stratmann (1891). A Middle-English dictionary. [London]: Oxford University Press. and A Concise Dictionary of Middle English from A.D. 1150 TO 1580, A. L. Mayhew, Walter W. Skeat, Oxford, Clarendon Press, 1888.
- English personal pronouns
- Generic antecedents
- Object pronoun
- Possessive pronoun
- Spivak pronoun
- Subject pronoun
- "'He or she' versus 'they'". Oxford University Press. Retrieved 2012-07-07.
- O'Conner, Patricia; Kellerman, Stewart (21 July 2009). "On Language - "All-Purpose Pronoun"". New York Times Magazine. Archived from the original on 7 July 2012. Retrieved 7 July 2012.
- "They". Online Etymology Dictionary. Retrieved 6 October 2010.