In the Greek, a distinction is made between the second person singular and plural. If you need a grammatical refresher, the second person is “you,” as opposed to first person (“I”) and third person (“he/she/it”). A distinction between singular and plural is important to indicate if the “you” is one person, or a group. For example, if I’m talking to my class, I could say “you need to pay attention” and be addressing the whole group (plural), or “you need to pay attention” and be talking to the sleeping guy on the front row (singular). As you can tell, in contemporary English, we use the word “you” for both. As I said, Greek did not. If it’s singular, it’s one way, if its plural, it’s another. The KJV translators, however, used the “thee’s” and “thou’s” to show the difference. They translated Greek second person singulars with “thee” and “thou,” and translated Greek second person plurals as “ye,” and “you.” That’s one area that makes the KJV a superior translation. While it’s not WRONG to translate the singular as “you,” it is LESS PRECISE. When modern translations leave out the “thee’s” and the “thou’s” they are not wrong, but they are neither as accurate nor precise as your Authorized Version.