In Old English (OE) the second person in singular and plural had a number contrast.
In the thirteenth century a change in the form of the second person started to happen. Next to the Old English form ‘Žū’, the form ‘you’ was more and more used for singular address. This might go back to the influence of the French court.
From the French language a system arose, known as ‘T/V’ system. T/V means Tu/ Vous. The pronoun T (in German “du”) was used in intimate situations and when the speaker wanted to show the listeners his or her solidarity. The V-form indicated power and status between the interlocutors, whereas Tu showed social equality. The use of ‘you’ allowed new possibilities of marking intimacy and contempt.
To assign this system to the English language one could assume that thou can be compared with the T-form, you with the use of Vous. This assignment would be too easy because the use of the both pronouns is much more complex.
In the English language a stable and differentiated use of the forms never developed. In the upper-class for example, V was used among the members for addressing each other. Whereas they used T (in English thou) to mark on the one hand, an asymmetrical status relation, which could be permanent as well as temporary, on the other hand to indicate an emotional and intimate tone in a talk or a letter. The use was also influenced by the topic, the relationship of the involved persons and other conditions. In the sixteenth and seventeenth century, besides the pragmatic use, a grammatical distinction in use arose. Thou was often used as a subject of auxiliaries, you was used with lexical verbs.
Auxiliaries are words with a grammatical function, such as can, will, do, etc. Lexical verbs are words of the lexical category verb, e.g. go, eat, etc.
From the fourteenth century on you developed to the term of singular address. This development spread into the seventeenth century. Grammarians of that period reported, that “In ordinary speech we say you for thou, thee and ye, but empathically, contemptuously or caressingly we say thou.” Over the years you and thou “became more pragmatically distinct”. (Lass 1999: 150)
To get information about the use of you and thou in Early Modern English is quite difficult because the only passed down documents that come close to a conversation are letters from that period. Another problem is that the written style of letters was quite formal in the fifteenth century. It was not common to use emotional expressions, even not between lovers or family members. Not until the beginning of the sixteenth century a clear difference in the use between you and thou could be found.
Letters of the seventeenth century contained a mixture of both forms. Which form was used, depended less on social factors, than on the topic or the emotional tone. “What seems to have happened in this late period […] is that the you vs. thou contrast finally became a deictic one”. (Lass 1999: 153) You, as the distal pronoun was used when the interlocutors talked about business, unreal conditions and social superiors. Thou, however, was the proximal pronoun the speaker used, when the topic was ”restricted to an immediate, factual or real present”. (Lass 1999: 153) In the end of the seventeenth century you was more often used, than thou. Interesting is, that the use of the pronouns differentiated among the members of the same social classes. The use was still optional. In the eighteenth century you became the normal form, thou was only used for “high-register discourse”.