Here's why:
* Pronoun case: "Them" is an object pronoun, used when the pronoun is the object of a verb or preposition. "You" is a subject pronoun, used when the pronoun is the subject of a verb.
* Subject-verb agreement: When using "them and you" together, you need to use a plural verb.
Here are some correct ways to use these pronouns:
* Subject: "You and they are going to the store."
* Object: "The teacher gave the assignment to them and you."
Let me know if you'd like to see more examples!