Even if those colleagues of yours are native English speakers, they are definitely grammatically wrong in how they compounded the subjects in “You and me are...”, wrong in how they compound the doers of the action in “Me and my friend are going to...”, and wrong again in how they compounded the objects in “Between you and I.” The correct way to compound them is, as you’ve pointed out, as follows:
Compound subject: “You and I are…”
Compound doers of the action: “My friend and I are going to…”
Compound objects: “Between you and me…”
It seems to me that your colleagues think they have the upper hand in grammar and usage simply because they are native English speakers, but they are wrong in that belief. They seem to be unaware of the importance of avoiding mixed-case usage in English. By being too colloquial and cavalierly with the language, they are obviously not practicing what they are preaching to their students. And if they have managed to confuse you with their errant grammar, they must be confusing their students as well. I just hope that this situation in your school will not go on uncorrected.
It will be a bit complicated to show that your colleagues are wrong, but let’s make the effort and go through the process now. I’m sure it will be worthwhile for you and for others who may not be conversant with the grammar rules involved here. So here goes…
The Matter of Case
Remember the matter of case in English? Case is any of the three forms that a noun, pronoun, or modifier takes to indicate its functional role in a sentence, whether nominative or subjective, objective, or possessive. We need to understand these case forms very well to be able to combine nouns and pronouns properly when constructing sentences.
1. Nominative or subjective case – Nouns or pronouns are in the nominative case when they perform the verb’s action (doer of the action), and are in the subjective case when they act as the subject of the sentence.
Example:
Nominative:
“The woman slapped him.”
[The noun “woman” is in the nominative case because it is the doer of the action]
“She slapped him.”
[The pronoun “she” is in the nominative case because it is the doer of the action]
Subjective:
“The woman is lovely.”
[The noun “woman” is in the subjective case because it is the subject of the sentence]
“She is lovely.”
[The pronoun “she” is in the subjective case because it is the subject of the sentence]
2. Objective case – Nouns or pronouns are in the objective case when they receive the verb’s action or act as the direct or indirect object of a sentence.
Examples:
“The woman slapped Mario.”
[The noun “Mario” is in the objective case, serving as a direct object of the verb “slapped”]
“The woman slapped him.”
[The pronoun “him” is in the objective case, serving as a direct object of the verb “slapped”]
“The woman slipped Mario a note.”
[The noun “Mario” is in the objective case, serving as the indirect object of the verb “slipped”]
“The woman slipped him a note.”
[The pronoun “him” is in the objective case, serving as an indirect object of the verb “slipped]
3. Possessive case – Nouns or pronouns are in the possessive case when they indicate who or what possesses or owns something.
Examples:
“This seat is mine while that one is yours.”
[Both “mine” and “yours” are possessive pronouns]
“Theirs is the glory while ours is the hard work.”
[Both “theirs” and “ours” are possessive pronouns]
As I’m sure you already know very well, the possessive case is actually the simplest of the three cases in English. They are virtually no-brainers so we need not take them up in detail here.
The Case Rule in English
Now, so we can properly compound nouns and pronouns or use them in combination as subjects, doers of the action, or direct or indirect objects, we must observe the so-called case rule in English.
Before discussing the case rule, however, we need to keep firmly in mind this very important characteristic of nouns in English: even if nouns do take a particular case when used in sentences, they don’t change form or inflect except in the possessive case; in contrast, the pronouns change form or inflect in all of the cases.
Specifically, nouns don’t change form or inflect at all in the nominative or subjective case as well as in the objective case. For instance, the noun “Elvira” will be “Elvira” in all those three cases. (In the possessive case, of course, the noun changes form or inflects to “Elvira’s.” That’s all.)
With this qualification about the difference between nouns and pronouns, we are now ready to take up the case rule in English.
The case rule: A noun and pronoun being used in combination to form a compound subject, a compound doer of the action, or a compound object of the verb should both be in the same case; otherwise, the sentence will be grammatically incorrect.
In practice, we don’t have to consciously apply the case rule in the following situations: when the compound subject of the sentence consists of both nouns, when the compound doer of the action consists of both nouns, and when the compound receiver of the action consists of both nouns. This is because nouns don’t inflect or change at all in all such cases, as I explained earlier.
It is actually when a noun and pronoun—or a pronoun and another pronoun—are combined to form compound subjects, compound doers of the action, or compound objects or receivers of the action that the proper application of the case rule becomes crucial. The case rule provides that we can’t mix a noun and pronoun—or a pronoun and another pronoun—that are in different cases. When we do, the resulting sentence construction becomes grammatically incorrect.
Here are examples of disallowed case mixing:
1. Mixing a noun and the objective-case pronoun “me”
Wrong:
“Jenny and me like each other.”
(The noun “Jenny” is in the nominative case but the pronoun “me” is in the objective case, resulting in case mixing.)
Correct:
“Jenny and I like each other.”
(Both the noun “Jenny” and the pronoun “I” are now in the nominative case.)
2. Mixing the nominative-case pronoun “you” with the objective-case pronoun “me”
Wrong:
“You and me should travel together sometime.”
(The pronoun “you” is in the nominative case and the pronoun “me” is in the objective case, resulting in case mixing.)
Correct:
“You and I should travel together sometime.”
(Both “you” and “I” are in the nominative case.)
This brings us to your colleagues’ mixed-case construction:
“Me and my friend are going to...”
(The pronoun “me” is in the objective case while the noun “my friend” is in the nominative case, resulting in case-mixing.)
Correct:
“I and my friend are going to…”
(Both the pronoun “I” and the noun “my friend” are now in the nominative case.)
Better still (as matter of good form):
“My friend and I are going to…”
3. Mixing the objective case “you” and the nominative case “I”
Wrong:
“Our neighbors are nasty to you and I.”
(The pronoun “you” is in the objective case while the pronoun “I” is in the subjective case, resulting in case mixing.)
Correct:
“Our neighbors are nasty to you and me.”
(The pronouns “you” and “me” are now both in the objective case.)
4. Mixing the objective case “me” and the subjective case “they”
Wrong:
“I wish they were nicer to me and they.”
(The pronoun “me” is in the objective case while the pronoun “they” is in the subjective case, resulting in case mixing.)
Correct:
“I wish they were nicer to me and them.”
(The pronouns “me” and “them” are now both in the objective case.)
I think we’re done now in clarifying the case rule in English. All we need to do now is to see if your colleagues’ usage of “Between you and I…” is correct. It’s true that some grammarians still insist that the use of the nominative case “I” in this phrase is correct, but it’s now generally accepted that the objective case pronoun “me” is the formally correct usage here, as in this sentence: “This secret is just between you and me.”
So, Dessang, you absolutely didn’t learn something wrong or miss out something in school. It’s your colleagues who are definitely wrong in their mixed-case constructions. They are evidently clueless about the case rule in English and it will be great if you can somehow bring this discussion to their attention—the sooner, the better.