In Modern English, you is the second-person pronoun. It is grammatically plural, and was historically used only for the dative case, but in most modern dialects is used for all cases and numbers.
You comes from the Proto-Germanic demonstrative base *juz-, *iwwiz from Proto-Indo-European *yu- (second-person plural pronoun). Old English had singular, dual, and plural second-person pronouns. The dual form was lost by the twelfth century, and the singular form was lost by the early 1600s. The development is shown in the following table.
Early Modern English distinguished between the plural ye and the singular thou. As in many other European languages, English at the time had a T–V distinction, which made the plural forms more respectful and deferential; they were used to address strangers and social superiors. This distinction ultimately led to familiar thou becoming obsolete in modern English, although it persists in some English dialects.
Yourself had developed by the early 14th century, with the plural yourselves attested from 1520.
In Standard Modern English, you has five shapes representing six distinct word forms:
you: the nominative (subjective) and accusative (objective or oblique case) forms
your: the dependent genitive (possessive) form
yours: independent genitive (possessive) form
yourselves: the plural reflexive form
yourself: the singular reflexive form
Although there is some dialectal retention of the original plural ye and the original singular thou, most English-speaking groups have lost the original forms. Because of the loss of the original singular-plural distinction, many English dialects belonging to this group have innovated new plural forms of the second person pronoun. Examples of such pronouns sometimes seen and heard include:
y'all, or you all – southern United States, African-American Vernacular English, the Abaco Islands, St. Helena and Tristan da Cunha. Y'all however, is also occasionally used for the second-person singular in the North American varieties.