English, like all languages, has changed over the years. In modern English, you only have to choose between “you” and “your” to talk about the person you’re speaking to. But English used to have different words for “you” depending on how the word fit into the sentence (just like Spanish and other languages). If it was the subject, it was “thou”: “Thou art beautiful.” If it was the object, it was “thee”: “I will send thee a letter.”