There’s too much misuse of these words to develop the full grisly picture in one little entry, and we’ll inevitably have to revisit the sorry subject. So we’ll just take one that makes the author physically cringe every time.
Never, ever, ever, ever say “Him and I did such-and-such” or “Her and I are going to the mall” or whatever. These are such grievous grammatical sins that one should hang one’s head in shame.
Say “He and I did such-and-such” or “She and I are going to the mall.”
The simple thought here is that if someone is doing the action, we say “he” or “she” is doing it. If something is being done to or for someone, we say “him” or her.”
She and I are friends.
He and I are in the same class.
She and Fred went to the movies.
He and Emma are the worst couple ever.
Her and I are friends.
Him and I are cousins.
Her and Fred hated the movie.
Her broke up with Emma, thank goodness.
Just keep saying “He and I” and “She and I” over and over until you can’t say anything other than that when beginning a sentence.