You may be surprised to find that the following sentence is nonstandard in American written English.
Nonstandard
Every semester, a student leaves the class without their umbrella.
The most common error in pronoun / antecedent agreement is to use a plural pronoun (in this case, “their”) with the singular word it refers back to (“student”) in an attempt to avoid gender-specific language. Recall that pronouns are words such as she, their, her, or him that take the place of a noun in a sentence.
This nonstandard sentence can be fixed by substituting “his or her” for “their” or by making the word it refers to plural so that it agrees with the plural pronoun.
Standard
Every semester, a student leaves the class without his or her umbrella.
Standard
Every semester, students leave the class without their umbrellas.
Errors of agreement are often made with possessive case pronouns, and they’re also made with subjective case pronouns like I, you, he, she, it, we, you, and they.
Nonstandard
If someone wants to join the club, they need to bring a check for five dollars.
In the sentence above, the singular indefinite pronoun someone is paired with the plural subjective case pronoun they. This nonstandard use can be fixed in a number of ways, some more awkward than others:
Standard
If someone wants to join the club, he or she needs to bring a check for five dollars.
Standard
If someone wants to join the club, that person needs to bring a check for five dollars.
Standard
If you want to join the club, you need to bring a check for five dollars.
Standard
If people want to join the club, they need to bring a check for five dollars.
All of the above are correct, but the first two are more formal and the last two are more informal. Consider your audience as you find ways of avoiding this common
error of pronoun / antecedent agreement.