My grammar isn't perfect, no one's is. Everyone uses the wrong word occasionally or spells something incorrectly, it's no biggy. But my biggest pet peeve about grammar is the grammar police. People who go around correcting people's grammar. No one needs them to jump up and say "it's whom!" ugh. But when the grammar police are wrong, it's even more annoying. This is a conversation I had with an acquaintance not long ago.
"Marsha is busy at 4, but she can drive there with you and me at 6."
"It's you and I, not you and me. Don't you have an English degree?"
"Uh, ya I do, and it's you and me."
"No, it's not."
And I hear this a lot. People are always so sure it's you and I or Mark and I, they say it all the time. I know what they mean, like I said it's no biggy, but.... sometimes it's actually you and me. I is a subject pronoun, when you and I are the subject of the sentence it's correct to use I. Me is an object pronoun. When you and me are the object, it's correct to use me.
For example: You and I need to study for our math test tomorrow. You and I are performing the action- to study and are therefore the subject.
But if the sentence was: Julie is going to help you and me study for the math test: Julie is performing the action- to help and is therefore the subject. You and me are the ones receiving her action and are therefore the object of the sentence.
When in doubt use the old trick of removing the other person and see if the sentence makes sense. Julie is going to help me study for the math test. Julie is going to help I study for the math test. The answer is obvious.