Since the wizards of education theory gave up on Latin as a “dead language” English grammar has declined. It doesn’t have to be that way. Learning Latin (or Greek) does improve English grammar but you can improve your English usage without learning Latin or Greek simply by making a simple distinction that you already understand.
Listen closely today. I’m confident that you will hear someone say, “Her and So and So did such and such” (or “Him and so and so…”). Why is this wrong? It’s incorrect because the forms he and she are in the subject case and her and him are in the object case. Subjects and objects are different things. They have different functions. Used correctly: “She threw him out of the window” or “He threw the frisbee to him.” Her and him are either the direct object of the action (i.e., something is done to them) or the indirect object. In the sentence, “He made dinner for her.” He is the subject. The dinner is the direct object. He acts directly upon the dinner (in making it and then later in eating it!). The person who received the dinner is the indirect object. They have two distinct roles.
This happens frequently with who and whom. Who is the subject and whom is the direct or indirect object. “Who called?” To express the person receiving the call use whom. “Whom did he call?” To express the person making the call use who. “Usain Bolt ran so fast he that defeated Fred, whom he has defeated 73 times consecutively.” If, in the subordinate clause beginning after the comma, we used who the sentence would become ambiguous.
Grammatically, he, she, and who are parallel and him, her, and whom are parallel.
If we distinguish between subject and object (which is important in other ways too) we will be clearer and avoid confusion.
Someone will object, “Don’t be pedantic. Language evolves. When someone says ‘her and me went to the mall.’ we know what they mean and that’s all that matters.” Yes, language does evolve but it also devolves into chaos and confusion. The most likely reason that one might say “him and me drank a beer” is that the speaker doesn’t know the difference between the subject and object. That ignorance is likely to manifest itself in other constructions that will be confusing.
The objection assumes that grammatical rules are arbitrary, that there is no real relation between reality and language. That’s a big and quite dubious assumption. There is a real connection between language and the way things are. There are subjects and objects and they are not the same thing. They do not ordinarily perform the same function. There are exceptions, of course, but that’s the great thing about exceptions: they test the rule but they aren’t the rule.
The old radio commercial was correct: people judge you by the words you use. When you use the object where the subject is wanted you sound less intelligent than you really are. Why shouldn’t we express the subject and the object clearly? We should we submit to the prevailing decline in the culture?