[jacobolus]

In Old English, thou was the singular second person pronoun and ye the plural, but would be used when talking to someone of higher status. From what I understand, the ye evolved into you while thou mostly disappeared, so now we all get to be plural. I’m not sure about third person pronouns though.

See https://en.wikipedia.org/wiki/T–V_distinction

[dragonwriter]

> In Old English, thou was the singular second person pronoun

Not just Old English; it survived in active, everyday use well into Modern English.

[mitchty]

Don't the Pennsylvania Dutch still use it? I vaguely recall there being a community out east that uses either thee or thou still with regularity.