"I sit here," said the teacher, "and y'all sit there." He did not forget to add, "But don't write y'all in your writing!"
The case of second person plural pronoun in English is the most fascinating. As you know, Modern English uses one word, "you", for both singular and plural form of second person pronoun. I believe I used the plural form in the last sentence. And "y'all" shows desire to make this distinction.
It wasn't always like this. Middle English used two separate words for singular and plural form of second person pronoun. Guess what they were? Singular form was "thou", and plural form "ye". Compare Modern German, which uses "du" and "ihr" respectively. And the distinction was present for many thousand years, since these pronouns ultimately originate from Proto-Indo-European root "tu" and "yus"!
Then how did English come to lose this distinction, that they have kept for such a long time? It is still a mystery to me, but one theory is that "ye" had started to be used as a "polite" form, analogous to French "tu" versus "vous". So for singular form of second person pronoun, "thou" became a "plain" form, while "ye" became a "polite" form. Later, "thou" started to connote contempt, and its use declined. Well, that is the theory. I don't buy it.
It is rather ironic that remaining use of "thou" in Modern English is to address God. "For thine is the kingdom, and the power, and the glory, for ever." Does this mean that we address God in plain, familiar, informal, even contemptous manner? No, it just means that liturgical language simply refuses to change, and that this English prayer was translated from Latin prayer when second person singular pronoun was alive and well.
After all, irregular plural forms are so European. We Koreans simply attach the plural suffix to pronouns. But we do have separate forms for plain and polite usage.