They

In Modern English, they is a third-person pronoun relating to a grammatical subject.

Morphology

In Standard Modern English, they has five distinct word forms:

they: the nominative form
them: the accusative and a non-standard determinative form.
their: the dependent genitive form
theirs: independent genitive form
themselves: prototypical reflexive form
themself: derivative reflexive form
History

had a single third-person pronoun He, which had both singular and plural forms, and they wasn't among them. In or about the start of the 13th century, they was imported from a Scandinavian source, in which it was a masculine plural demonstrative pronoun. It comes from Proto-Germanic *thai, nominative plural pronoun, from PIE *to-, demonstrative pronoun. According to The Cambridge History of the English Language:
The development in Middle English is shown in the following table. At the final stage, it had reached its modern form.

	I	II	III
Nominative	þei	þei	þei
Oblique	hem	hem	hem ~ þem
Genitive	her	her ~ þeir	þeir

Singular ''they''

is a use of they as an epicene pronoun for a singular referent. In this usage, they follows plural agreement rules, but the semantic reference is singular. Unlike plural they, singular they is only used for people. For this reason, it could be considered to have personal gender. Some people refuse to use the epicene pronoun they when referring to individuals on the basis that it is primarily a plural pronoun instead of a singular pronoun. However, the online edition of the Oxford English Dictionary records usage of they "referring to an individual generically or indefinitely", with examples dating from a1405–2019.

Word of the year

In December 2019, Merriam-Webster chose singular they as word of the year. The word was chosen because "English famously lacks a gender-neutral singular pronoun to correspond neatly with singular pronouns like everyone or someone, and as a consequence they has been used for this purpose for over 600 years."

Syntax

Functions

They can appear as a subject, object, determiner or predicative complement. The reflexive form also appears as an adjunct.

Subject: "Theyre there"; "them being there"; "their being there".
Object: "I saw them"; "I directed her to them"; "They connect to themselves."
Predicative complement: "In our attempt to fight evil, we have become them"; "They eventually felt they had become themselves."
Dependent determiner: "I touched their car"; "them folks are helpful".
Independent determiner: "This is theirs."
Adjunct: "They did it themselves."
Dependents

Pronouns rarely take dependents, but it is possible for they to have many of the same kind of dependents as other noun phrases.

Relative clause modifier: "they who arrive late".
Determiner: "Sometimes, when you think, 'I will show them', the 'them' you end up showing is yourself."
Adjective phrase modifier: "the real them".
Adverb phrase external modifier: "not even them".
Semantics

Plural theys referents can be anything, including persons, as long as it does not include the speaker or the addressee. Singular they can only refer to individual persons. Until the end of the 20th century, this was limited to those whose gender is unknown.

Generic

The pronoun they can also be used to refer to an unspecified group of people, as in "In Japan they drive on the left", or "They are putting in a new restaurant across the street." 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."