In English, as in other languages, we employ "First Person" (the one speaking), "Second Person" (the one being spoken to), and "Third Person" (the one being spoken about). There is singular (I speak) and plural (we speak). Using French as an example, I can say tu parles to my friends (Second Person Singular) but to show respect I would say vous parlez, whether to one of you with whom I'm not familiar, or to more than one of you. So vous is the plural as well as the formal form of tu. The same in English. You is the plural of thou. Other languages use either she and the Third Person to show respect since such words as grace and majesty are feminine (Sie in German and Lei in Italian) whereas others use a word formed from Your Grace to show respect (Vuestra Merced in Spanish becomes Usted after Vuestra being abbreviated as "Vst" which became "Ust" and combined with "-ed" from merced. The Portuguese você has a similar construction coming from vossa mercê, or Your Grace.) In some Spanish-speaking countries, the formal Usted (which, keep in mind takes the Third Person form) replaced the Second Person tú. This is what happened in English: the formal use of you, even in the singular sense, replaced thou almost everywhere, except for example among the Amish: thou speakest and you speak. The same for their object pronouns thee and ye which were replaced by the subject pronoun you. So that's what happened to thou and its suffix -est.
As for the ending -(e)th, that was replaced over time by -(e)s: he maketh becomes he makes.