Overview
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.
History
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 and the singular . 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.
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