English used to be like other European languages too. We had thou/thee for singular, and you/ye for plural, and for respectful singular. Eventually, people began using it as respectful singular for everyone, and so it just became singular and plural, eclipsing thou/thee. Around this time, the you/ye accusative/nominative distinction was also lost, so now we just have you.

If you're curious, the you/ye distinction worked like this: "you" was used for the subject (the doer) of the sentence, and "ye" was used for the object (the done to). you/ye are analogous to I/me.

"You come with me." (plural you)

"I come with ye." (plural ye)

As a result of the loss of thou, we also lost the conjugation of verbs related to it, like "art" instead of "are", and "-st" or "-est" for other verbs ("goest", "thinkst", etc). It used to be that "are" was only for plural pronouns, but now both "you" and "they" can be singular.

And if you're curious about what happened to "-eth", evidence suggests this was for a long time a typographic feature, and it was pronounced "-s" as it is today. It was used exactly like "-s". "He thinketh" would have been pronounced "he thinks".