A word-for-word swap into early modern English: you becomes thou, why becomes wherefore, and before becomes ere.
The most recognisable feature of early modern English is a pronoun distinction modern English has lost. Thou was the singular and informal you, used with family, close friends and inferiors; ye or you was plural or formal. Thou is the subject, thee the object, thy the possessive before a consonant and thine before a vowel. Verbs agreed with it too, which is where art, hast and dost come from. The distinction was already fading in Shakespeare's lifetime, and he uses the switch between thou and you deliberately, characters change pronoun mid-scene as their relationship shifts.
This is a word-for-word substitution, and that is a real limit worth stating. Some mappings are context-dependent and cannot be done correctly by a lookup table: mine belongs before a vowel and my before a consonant, so those are deliberately left alone rather than applied wrongly everywhere. Verb agreement is only right when the subject is thou. Real early modern English also differs in syntax and vocabulary in ways no substitution reaches. The output is a recognisable pastiche, which is the honest description of what this kind of tool produces.
Thou is the subject and thee the object, thou speakest, I tell thee. Thy is possessive before a consonant, thine before a vowel.
Because some swaps depend on context. Mine belongs only before a vowel, so applying it everywhere would produce wrong forms like mine good fellow.
It is a recognisable pastiche. Genuine early modern English differs in syntax and vocabulary in ways a word-for-word swap cannot reach.