Grammar is a combination of rules and conventions. What’s the difference? Well, there are the rules, like a verb must agree with its subject. By that rule, “he say” is incorrect. Then there are conventions, which are uses of language that are common enough that even though they break the “rules” they become “correct” simply through repeated usage. There are other conventions that vary from place to place, but that’s a much bigger discussion!
In the introduction to the 2003 edition of The King’s English, Matthew Parris reminds us that “There is no authority. English is not a managed language. Nobody is in charge.” Over time, English speakers themselves become the authority. Some accepted conventions sound very natural, like saying “I’m good” instead of “I’m well.” Through their ubiquity, they’ve become an accepted part of the language.
So, what about “between you and I”? Technically, it should be “between you and me.” However, the phrase “between you and I” has become accepted as an idiom of its own. Even Shakespeare used it! Confusing “me” and “I” is one of the most common grammar problems. Using the word “I” can sound learned and elite; however this leads to it being overused when it’s actually incorrect. This problem is called hypercorrect incorrectness.
The “you and me” problem is more confusing when there are two objects, like in the sentence “Thanks for inviting my husband and I to dinner.” If you’re ever unsure, here’s a simple trick. Omit the first person mentioned and see how it sounds. If you said, “Thanks for inviting I to dinner,” it sounds wrong. Whoops. Without two people, it is easier to use your ear to hear if “I” or “me” is grammatically correct.