Apart from the dialect form used in the Southern US, "y'all," has English ever had a plural "you"? If not, how does English get around using this form?
You and ye used to be the plural forms of the second person pronoun. You was the accusative form, and ye was the nominative form.
Because of this, you still conjugates verbs in the plural form even when it is singular; that is, you are is correct even if you is only referring to one person.
Thee and thou used to be the singular forms. Thou was the nominative form, and thee was the accusative form. Thy and thine were the genetive forms, and their use followed the same rules as a and an.
You is the plural.
Thou is the singular form of you.
Thou has now disappeared from common use and is used only to address God.
The process resulting in the use of the singular pronoun to express intimacy and the plural pronoun to mark respect or social distance is termed T-V_distinction, after the Latin tu and vos and is found is many languages, especially of the Proto Indo European family tree.
See for instance, in addition to the Latin form above:
- French: tu => vous
- German: du => ihr (2nd person plural) or Sie (3rd person plural)
- Mandarin 你 (nǐ, you informal) => 您 (nín, you respectful) compared to (nǐmen, 你们, you to several persons).
Even some languages that seem not to comply exactly (because they don't seem to use the 2nd-person plural) actually hide a form a compliance.
- Spanish: tu => vos (obsolete, 2nd-person plural archaic form).
- Italian used to use voi (2nd person plural).
- The você of Portuguese is a contraction of vossa mercê (your mercy) which is an implicit 2nd person plural.
All in all, English has pushed T-V distinction so far that thou is not used anymore in common speak.
'You" was originally plural, "thou" was the singular.
There was a shift to using the plural as the polite form, eg. monarchs say 'we' for I, so gradually the 'you' plural began to be used by everybody.
Exceptions are/were Quakers who stuck to the thee/thou since they didn't recognise anyone as better than each other and people from Yorkshire who didn't recognise anyone as better than them.
In middle English, there were informal terms for "you" singular (thou/thee) and informal "you" plural (ye/you). So for awhile there, in Chaucerian times, you could make the distinction grammatically. The formal 2nd person pronoun was "you" for both singular and plural.
In the transition to modern English, for politeness' sake or simplicity, we got rid of the formal/informal inflections and just went with the formal pronouns. It really isn't a problem because context and verb endings will almost always make clear whether singular or plural is meant (there's no grammatical need for a "y'all").
Note that in other languages like Japanese, we see that pronouns can often be dispensed with entirely.
As others have said, Thee and Thou were the informal terms. But in a bizarre switch, Thee and Thou are now considered more formal. I see two forces at work to bring this about.
First, early English bibles (at least the King James edition) used Thee and Thou when addressing God, to emphasise a personal, informal relationship with God. Nowadays, the relationship to God is considered more formal, and the old bibles themselves are considered formal, so when people today use Thee and Thou to refer to God, it feels to be a formalism.
Second, thee and thou are used in Shakespearean writing. People have a respect for Shakespeare, and afford a certain formalism to all his writing (even the parts that are designed to be informal). So there again, thee and thou seem Shakespearean and therefore more formal.
So now it's switched, with thee and thou feeling more formal, and you seeming less formal.