Your teacher always told you to say “so and so and I” instead of “me and so and so”. This is correct (most of the time), but did they explain why? It is often a point of confusion for most people because while teachers usually remember to teach that, they often do not explain the reason for it, which leaves people not understanding, which in turn can lead to problems such as using it incorrectly or not at all.
Many people think (and some are even taught!) that it is just more polite to put the other person first but that is NOT the reason. There is a legitimate grammatical reason.
In a proper English sentence, the first-person singular pronoun I is used where speaker is the subject of the sentence. That is, when the person is the one doing something. When the person is the object in the sentence, me is used instead. This is when the person is having something done to them. Examples of the former include I went there and I didn’t know what it was. Examples of the latter include He gave it to me, They helped me, and It happened to me too.
That’s great, but there is a much easier and faster way to determine whether a given sentence is grammatically correct. Since “blah and I” or “me and blah” are conjunctions (they combine two objects), you can break it down to its constituent parts to see if it still works. This is a simple method to show the proper usage. When do you use “…and I” and when do you use “…and me / me and…”? It’s simple; just separate the compound sentence by rewriting it with each subject, or object as the case may be, then test the sentences with each individually.
For example: “Bob and I saw the movie” becomes “Bob saw the movie” and “I saw the movie”. Both of those work fine, and so the sentence is fine as is.
What about “Me and Bob saw the movie” (or “Bob and me saw the movie”)? They become “Bob saw the movie” and “Me saw the movie”. Clearly, “Me saw the movie” is incorrect and so should be changed to “Bob and I”.
That is the reason you say “Bob and I”. Technically, you could also say “I and Bob” in a sentence like this, but most people would agree that it sounds and feels pretty awkward: “I and Bob saw the movie”
However, you do not always use “…and I”. For example, “You saw Bob and I” is incorrect because breaking it down becomes “You saw Bob” which is fine and “You saw I” which is wrong. The correct sentence would be “You saw Bob and me” or “You saw me and Bob”. This breaks down to “You saw Bob” and “You saw me” which is correct. At this point, the order of the subjects is up to the writer. Both “You saw Bob and me” and “You saw me and Bob” are valid, and there are no official rules to their order.
One hiccup comes in the form of the possessive personal pronoun, namely “my” or “mine”. This does not really work in the same way, though it is somewhat easy to figure out by simply trying the different forms. It would be “His any my cars were parked” as opposed to “His and mine cars were parked” (alternately, though somewhat awkwardly, “My and his cars were parked” instead of “Mine and his cars were parked”). Similarly, “They liked his and my food” is correct where “The liked his and mine food” is not. Where “mine” is correct, it is rarely combined due to its usage: “it was mine and his” (or “it was his and mine”) is usually just written as “it was ours”. Note, this can also be applied to the other forms as well: “We saw the movie”, “You saw us”, “Our cars were parked”, and “They liked our food”.