Learn English – Which English dialects have 2nd person plural

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.

The Southern USA is one area where the pin-pen merger is common. It's not restricted to "pin" and "pen", but a lot of words that include /ɪ/ and /ɛ/ before nasals:

The pin-pen merger is a conditional merger of /ɪ/ and /ɛ/ before the nasal consonants [m], [n], and [ŋ]. The merged vowel is usually closer to [ɪ] than to [ɛ] (examples include: kin-ken, bin-ben, and him-hem). The merger is widespread in Southern American English, and is also found in many speakers in the Midland region immediately north of the South, as well as in less densely populated inland areas of the Western United States, particularly in Bakersfield, California. It is also a characteristic of African American Vernacular English.

Language Log has also discussed this, and examples where the vowels are pronounced the same and differently in a dialect with the merger may also shed some light on the phenomenon.