Thou or you?
Old English had both plural and singular second person pronouns. By Late Middle English, the singular, second person pronoun had evolved into "thee" and "thou", while the plural, second person had evolved into "you" and "ye". The story of the loss of "thee" and "thou" and its replacement with "you" in Modern English is not an entirely clear one; through various hypothesis and educated guesses, we can piece together some ideas about what might have caused this loss.
By the thirteenth century, you was sometimes replacing both "thee" (used in the object position) and "thou" (used in the subject position) among the upper classes. It might at first appear that these pronouns were being used interchangeably and at random but closer inspections reveal a probable semantic distinction between the two pronouns. Many scholars suspect that the English were trying to create an English form of the second person address similar to that of the French system ("tu" and "vous"). In this system, "vous" was used not only as a plural pronoun but also as a singular pronoun as a sign of respect when addressing one’s superiors and equals, while "tu" was used to address social inferiors as well as those with whom the speaker (or writer) shared some intimacy. It is generally thought that, initially, "you" was used in English similarly to "vous" in French, while "thee/thou" was used similarly to the French "tu".
However, it appears that, by the time of Early Modern English, or perhaps even Late Middle English, "you" became the neutral or unmarked second person singular pronoun while "thee/thou" was marked, at least among the upper classes. The two pronouns no longer functioned as a strict TV-system, if, indeed, they ever had. By the seventeenth century, "thee/thou" seems to have been used most often to denote either "intimacy (if used reciprocally) or contempt (non-reciprocally)" and the connotations of choosing "thee/thou" over "you" seem to have had a less direct correlation to social status.
Many of the conclusions drawn about the distinction between "thee/thou" and "you" are drawn from a careful study of literature from the various periods in question, most notably Sir Gawain and the Green Knight, as well as texts from Chaucer, Malory, and Shakespeare. Through observations of context (social status of the character speaking, tone of a situation, levels of intimacy, emotional qualities, etc.) in which the authors chose to use the marked instead of the unmarked second person pronoun, scholars are able to make some generalizations about the semantic differences between "thee/thou" and "you". However, there are some problems with this method of studying the "thee/thou", "you" distinction.
Firstly, the form of a particular genre may typically call for one or the other pronoun. For example, courtly romances generally used "you" throughout, while Elizabethan sonnets tended to lend themselves to the use of "thee/thou". While some of these formal variations may be attributable to the differences in tone in various genres, it’s not safe to assume that they are. For example, one might expect Shakespeare to have used more T forms in his comedies, with their lower class characters, but this does not prove to be the case.
The whims of individual authors can affect their decisions about when and how to use "thee/thou" and when and how to use "you". As Norman Blake points out, there may have been differences between "thee/thou" and "you" which authors could have exploited to convey subtleties of tone, mood, and situation within their works but, "The problem is to know whether a particular author does use these forms in a meaningful way" (537). If we extrapolate understandings of the "thee/thou", "you" distinction form literary texts, we are again left to hypothesize without ever having the certainty of knowing if our hypothesis are correct.
We must also remember that literary works may not accurately reflect how language was actually used during any given period; this is especially true of language as it was spoken. Many artistic and individual considerations may have affected authors’ word and syntactical choices. Even if they were attempting to portray casual or spoken language in an authentic way in their work, they may not have been skilled enough to do so. In addition, until relatively recently, those who were literate were those within a limited elite within a society. Their writing may accurately reflect upper class usage but may not accurately reflect those of the lower classes.
In an attempt to address this situation, scholars have turned to private correspondences for something more akin to private speech patterns, though this still does not address class issues. Here we do find evidence that "thee/thou" was used in discourse of a more intimate tone but, of course, letters between friends and family members tends to be more intimate by nature so, again, conclusions are based upon incomplete information.
More recently, scholars have turned to late sixteenth century transcriptions of "authentic speech from [court] witness depositions" to get a sense of both authentic and lower class speech patterns. Here, they have found that "thee/thou" is not used as a class marker but is used to express "anger or scorn" toward the addressee.
By the eighteenth century, "thee/thou" had been almost completely supplanted by "you" and "is not really a living option in ordinary usage... by the middle of the eighteenth century you was the only normal spoken form" (Lass, CHEL, V.III, 153). Yet "thee/thou" did survive in a few specific situations. Some poets continued to use "thee/thou", perhaps as an attempt to emulate the language of the Elizabethan and Jacobean poets.