My English Teacher’s pet peeve is mismatched items. She hates it when a writer uses past and present tense when referring to the same event, or when they mix up plural and singular forms.
Mismatched plural and singular is done so often, most readers don’t even notice it. Did you notice when I used the plural pronoun “they” to refer to the singular “a writer” in the first paragraph? I bet you didn’t. Properly, I should have used “writers” instead of “a writer”.
The pronoun “they” is often used incorrectly with a singular noun simply because the writer doesn’t want to offend the readers by implying gender. “They” is not gender specific; therefore it can refer to anyone who is reading the paper. However, many writers forget that “they” is also plural, and should only be used when referring to a plural noun.
If you have specified that there is only one person in your example, then you must either assign them a gender so that “he” or “she” can be used, or you must write out “he or she” or use the awkward slash “he/she”. This also applies to his/her/them and other variations. Commonly in English, the singular comes with a gender while the plural is not gender specific.
One way I’ve seen writers work around this problem is to use a male pronoun in one paragraph and a female pronoun in the next; however, personally, I find that more difficult to read than if the author simply picked a gender and stuck with it.