I have seen so many mistakes of this kind lately, in three or four different published novels plus several student papers. Probably this is a confirmation bias sort of thing — probably I’ve been sensitized recently so that these errors are leaping out at me more vividly and memorably than usual — but this is a common mistake in both writing and casual conversation. Since I’ve noticed it over and over in the past week or two, I thought I would take time to address it.
“You should vote for Samantha and I for class presidents!”
“That movie was terrifying — it really scared my mother and I!”
“That class is driving students to distraction, including my brother and I.”
In every case, the “I” should be “me” because the pronoun is part of the object of the sentence, not part of the subject. Those faint memories of teachers insisting on the “My mother and I” format are memories of compound subjects — or result from your teacher being clueless about grammar, though I hope that wasn’t the case.
Subjects are he, she, I, and we.
Objects are him, her, me, and us.
If you weren’t using a compound object, the mistake would be blazingly obvious, like so:
“You should vote for I for class president.”
“That movie was terrifying — it really scared I.”
“That class is driving students to distraction, including I.”
You see? Absolutely, obviously wrong. The quick and easy way to check, therefore, is to take the other person out of the sentence and see if you still want to use “I.” If not, then you should use “me” — even when you include the other person.
If this is something you can’t yet do reliably by feel, then I suggest you do a global search for “and I” and check every single usage in your manuscript.
A related error is this:
“Please keep that secret between you and I.”
Between is a preposition. You wouldn’t say to someone, “My brother was born two years after I.” When a pronoun comes after a preposition or is used as part of a prepositional phrase, it’s objective.
The correct version is: “Please keep that secret between you and me.”
“Between you and I” is so common that probably you can get away with it. Which is to say, as a writer, you can get away with it in dialogue when the character is speaking casually, but not when your character is an English professor or someone who normally speaks in a formal manner. Grayson Lanning, for example, would never say “between you and I.” If your character is supposed to be a pedant or formal, then you as the author need to be able to put formally correct phrases into that character’s mouth in order to encourage reader buy-in.
Similarly, you can get away with more errors in a novel that is written in an informal, light style than in high fantasy.
So when you see a discussion about grammar that says, Oh, whatever, it doesn’t matter, most people accept “between you and I,” you need to understand that in your life as an author, this just isn’t true. Your understanding of what is formally correct matters a lot because it increases the range of characters you can write believably and the overall range of styles in which you can write.