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thou/you is formal and informal and the distinction largely depending on your relationship with the person and respective social ranks. There were times when on

• tsimionescu 2y ago
  No, initially thou was simply the singular, with you as the plural second person pronoun. You'd address one person as thou, a group of people as you (like some speakers use you vs y'all today). Thou, my friend vs You, my friends. Then, under French influence probably, the plural, you, started being used as a polite form as well (in French, like most romance languages, formal/polite language uses the plural form of pronouns and verbs when addressing a single person). Thou, my friend VS You, sir; similar to "toi, mon ami" vs "vous, monsieur". Then, this polite form using singular you became so widely used that thou was almost entirely dropped, especially since English also had little distinction between singular and plural in verbs in general. You, my friend, you, my friends. Then, as thou became more foreign to regular speakers, it briefly started being used as a polite form, essentially reversing the original meanings. You, my friend VS Thou, sir. This didn't last very long, so finally we ended up with the current state, where there is no polite form and you is the only second person pronoun. Except of course some speakers have started using y'all for a plural form, but that doesn't seem to be gaining any popularity outside a few areas.