The question was, when do you use “you and me” and when do you use “you and I”. I’ve had a lifetime of haphazardly using whichever I felt like, but according to Babel there is a right time to say “you and me” and a wrong time to say “you and me”

All you have to do is remember which are the subject forms and which are the object.

(I/Me) went to school. (I)
(Him/He) went to school. (He)

You can’t say “Me went to school” and you can’t say “Him went to school”, so don’t say “Me and him went to school” or “Tim and me went to school”. It’s “Tim and I went to school” and “He and I went to school”. It’s usual to put ‘I’ in the last place. You *could* move it but it would sound odd in the same way that “fork and knife”, “wife and husband”, and “pepper and salt” would.

Unfortunately, because of this wrong “me”, some people hypercorrect in replacing “me” with “I” in the wrong places.

For (I/me). (Me)
For (he/him). (Him)

So “For Tim and I” (which is what you had written) is a hypercorrection. Prepositions “govern the accusative” in English. That sounds formal, but none of us would ever say “Is it for I?”, so we all know it, even if we can’t name it. Yet it’s so common to hear “Between you and I”. Common doesn’t make it right though.

So you have to know which form to use. If in doubt, just use one person (I/me) and look at which one works.

You wouldn’t say “Me has just been on holiday” so you mustn’t say “Me and Tim have just been”.

Similarly, you wouldn’t say “He bought it for I” so you mustn’t say “for Tim and I”.

It’s all very regular. I can’t see how people get it wrong, but maybe that’s just me being a snob

He is, admittedly, a snob I think grammar is rather like having sex education from nuns; if your school omit to teach you something, you spend years in complete ignorance of it, and it’s pure coincidence at what point in your life you accidentally discover that you’ve been totally misinformed….