A reader asked for free grammar helpwith this sentence: When a professor gives a quiz, they often catch their students by surprise.
The problem with this sentence is not really in the sentence itself, it’s in the English language. The subject “a professor” is singular; therefore the pronoun in the dependent clause must be singular as well. But English doesn’t tell us the gender of the professor. The traditional way to write this sentence is to use “he” for the dependent clause, and have the verb in the third person singular:
When a professor gives a quiz, he often catches his students by surprise.
However, many people object to this on grounds of sexism, so they write the sentence as you have, using “they” in place of a third person gender neutral term. But strictly speaking, that is not correct, according to accepted rules of English.
The alternative is to write in all possibilities:
When a professor gives a quiz, he or she often catches his or her students by surprise.
That’s a bit of a mouthful, and this solution, when used in an essay, could result in a paragraph chock full of “he or she” and “his or her” expressions. That can’t possibly be good writing.
I usually try to rearrange the sentence to remove the sexist bias, the gender ambiguity, and the awkwardness.
Students are often caught unawares by professors’ quizzes.
However, this sentence could be criticized for being in the passive voice.
You could simply change the number of the subject:
When professors give a quiz, they often catch their students by surprise.
This allows us to maintain the gender ambiguity without mixing plural and singular.