Let's start with a rather ordinary sentence: "My aunt took you and I to the theatre."
Many new writers might be surprised to learn that this is BAD grammar.
How can 'You and I' ever be BAD grammar?
How, indeed. Many of us spent our childhood being told by parents and teachers that we must stop saying "You and me" and say :You and I" instead. It is so drilled into us that to say "You and me" starts to sound wrong under any circumstance and we shy away from it. It is the same for a name followed by the pronoun, as well: Bill and me; Jenny and me.
When is 'You and I' correct?
"You and I" will always be correct when the pair of people referred to represent the SUBJECT of the sentence... the people who DO something. That usually means they start the sentence.
For example, You and I will go to Africa one day. Jack and I went to Starbucks for coffee yesterday. Alex and I think you should resign. You and I are still good friends after twenty years.
Testing the Validity
There is a very simple way to test whether the combination 'You and I' is correct. You just split the pair and say each one separately.
"You will go to Africa one day." " I will go to Africa one day."
"Jack went to Starbucks for coffee." "I went to Starbucks for coffee."
If someone is tempted to say: "You and me will go to Africa one day." then applying the same test will reveal why this is wrong. Let's split the pair.
"You will go to Africa one day" (That's OK) "Me will go to Africa one day." (Nope!)
When is "You and me" correct?
'You and me' will always be correct when the pair of people referred to are the OBJECT of the sentence ... they have something done to them - or for them - by someone who is the subject of the sentence.
Hence, we will write:
"Barry told Bill and me that he was getting married."
"The captain ordered Abigail and me to leave the ship."
"My aunt took you and me to the theatre."
Testing the Validity of "You and me."
When we apply the test above to 'You and me' you can quickly see that 'me' is correct.
"Barry told Bill that he was getting married." AND "Barry told ME that he was getting married."
"The captain ordered Abigail to leave the ship." AND "The captain ordered ME to leave the ship."
"My aunt took you to the theatre." AND "My aunt took ME to the theatre."
Conclusion
To return to our opening sentence which I declared wrong: "My aunt took you and I to the theatre.", when we split the pair, we get: "My aunt took I to the theatre.".. and no one would be likely to say that!