I can tell you which pronouns to use, but it takes a linguist to explain why people get confused. Here, Gretchen McCulloch reveals the fascinating reason that people struggle with sentences such as Billy and me went to the store.
A recent episode of the Autobiography of Jane Eyre (which is a fantastic webseries that has nothing to do with linguistics but you should check out anyway) provides an excellent example of the difficulties people have with you and I versus you and me.
The relevant portion is in Episode 59, between 2:50 and 3:13. Here’s a transcript:
Diana: If you need anything, just let Mary or me know.
Mary: It’s “Mary or I”
Diana: It’s “Mary or me”
Mary: No, it’s “Mary or I”
Jane: Actually, I think it’s “Mary or me”
Mary: You’re right! Cause if I wasn’t here you wouldn’t say “let I know” because that wouldn’t make any grammatical sense.
Diana: Anyways, if you need anything just let me know, and I’ll make Mary over here get it for you. (laughs)
The logic that Mary uses to pick between I and me is fairly standard grammarian advice (drop the Mary or the you and then use whichever one sounds right when left alone), but this leaves us with an important question: given that picking between I and me is intuitive in every other context, why do our intuitions abandon us as soon as you add an and?
In Latin, Subject Pronouns Go in the Subject Position
This question has a two-part answer. Part one is that grammarians have long railed against constructions like Me and Mary are going to the store favoring instead Mary and I are going to the store. The basis for their argument is that someone who is going to the store is in subject position, and therefore should use the subject form of the pronouns (such as I), regardless of whether there’s an and or not. It works in Latin, so it should work in English. So, if you get corrected enough times, eventually you might learn to say Mary and I or you and I instead of me and Mary or me and you.
However, for a lot of people, the rule that sticks is, “___ and I” is always better than “me and ___”. So they end up applying it to every context of “me and ___” or “___ and me”, even when the me isn’t in the subject position and therefore has no reason to be changed to I. This is an example of hypercorrection, which is also responsible for things like the scattershot use of whom whenever someone wants to sound more fancy.
Part two is, why does anyone ever say me and Mary or me and you in subject position in the first place? You might get a very young child or very beginner-level second-language speaker saying me is going to the store (me am going to the store?), but never a full-grown adult native speaker. And yet the same adults who have been speaking English their entire lives can produce Me and Mary are going to the store without a second thought, especially in a casual context where they aren’t thinking about which form to use. What gives?