One of the major grammar mistakes I see when line editing is matching plural and singular sentence subjects with their respective plural and singular pronouns. These are easy mistakes to make but they are also easy mistakes to correct.
If a subject within a sentence is singular, meaning the sentence is talking only about what one person, place, or thing did, the pronoun must be singular as well. If the subject is gender-neutral, use it. If the subject is male, use he or his; if female, use she or her; if it has a gender but you don’t know or it doesn’t matter if the subject is male or female, use she or he or his or her. Granted, writing he or she all of the time can make the piece feel cluttered and reading tedious so it is tempting to use they or their. Don’t do it. If you are writing he or she too many times, make the subject plural so you can use they, their, or them, etc. properly.
As mentioned, perhaps the easiest way to get around the whole he or she dilemma is to make your sentence’s subject plural and use the more easily-read they or its derivative. Sometimes, however, it won’t be so clear cut. Perhaps the subject to which your pronoun refers isn’t contained in the same sentence. The rule is still the same: if the subject to which you are referring is plural, use the they pronoun. For example, from Missing Emily: Croatian Life Letters on page 20 with underlining to hopefully make it clearer which pronouns go with which subjects:
“Within a few hours, I knew the whole story. Aunt Shari had taken Emily for a walk in her stroller to the park in her neighborhood. When they left to walk back home, Aunt Shari buckled Emily into her stroller. When they reached the intersection a block away from the park, Aunt Shari pushed the street light button and waited for the walk signal. The moment she stepped into the intersection, an unlicensed sixteen-year-old girl riding with her friends swerved around the cars stopped at the red light and struck Emily’s stroller.” (“They” refers to Aunt Shari and Emily.)
It can also get tricky when your sentence has multiple subjects or contains some descriptive words between the subject and verb. The rule is still the same: match the verb to the subject about which you are writing. They, them, and their also applies to inanimate objects or places as well as gender-neutral subjects, which also can make them easier to deal with. But in the interest of interest, change your work up at times. Find a synonym for your subject or use the he or she (properly) occasionally.
Of course, if you are talking about only yourself, use I. If you are including friends in your sentence talking about yourself, use the collective we. The good news is if you are talking directly to someone else (second person), you is proper whether you are addressing one person or a whole crowd.
Even if you know this rule about singular or plural subjects and pronouns, it is easy to slip up when you are cranking out a draft, making it that much more important when it comes time for line editing so read your work carefully and think through your sentences to ensure your grammar is correct.