When I was homeschooling my children, I made them learn and chant the subjective case pronouns (I, you, he, she, it, we, they) and the objective case pronouns (me, you, him, her, it, us, them) and learn when to use which ones. I bet they can all still do it. (Bonus points to the child who can also chant all the helping verbs.) You use the subjective case ones when the pronoun is a subject or a predicate nominative, and the objective case ones when the pronoun is an object (either direct, indirect, or the object of a preposition).
Everyone knows what a pronoun is. It's a word that takes the place of a noun, so you don't have to sound ridiculous like this:
Bob rode Bob's bike five miles to Bob's house.
Instead, you can sound intelligent and say:
Bob rode his bike five miles to his house.
In the second sentence, his is the pronoun, and Bob is what we call the antecedent, or the noun to which the pronoun refers. I think Bob needs a car, but that's another story for another day.
The funny thing about pronouns and antecedents, though, is that they must match in number. Singular antecedent, singular pronoun. Plural antecedent, plural pronoun.
You wouldn't say:
Bob rode their bike five miles to their house.
would you? It doesn't even sound right. Yet I see this error all. the. time.
Take the letter I got from Liberty University (the school I am currently paying $25,000 a year to educate my daughter) today. Here's how it begins:
Over the years, you have invested in the life of your child. As the time comes for you to guide them . . .
Wait, what? THEM? I thought we were talking about my CHILD (singular). Why would we use a plural pronoun?
The $25,000 answer is: we wouldn't. We would use one of the singular pronouns, her or him. Since this is obviously a form letter, we would say, As the time comes for you to guide him/her . . .
Unless you are the parent of twins, in which case you would be out of your mind sending them to Liberty anyway. That would be $50,000 a year.
Go forth and speak correctly.
Be thankful ~