WELL, TEACHERS TOO sometimes commit them. And these errors occur not just in students’ papers but also during recitations and in answering essay questions during exams.
Tim O’Keefe of University of Minnesota at Morris, in his article “Some Common Grammar and Usage Mistakes in Undergraduate Philosophy Papers” (© 2001 Tim O'Keefe), explains how the following terms are properly used:
1. “They” and “their”
This sentence is grammatically incorrect: “Billy exclaims that someone has to be responsible for their actions, they have to be accountable to their own deeds.” O’Keefe explains that “’they’ and ‘their’ are plural pronouns, not singular ones.” Since “someone” is a singular pronoun, “their” and “they” are not supposed to be used in the sentence. O’Keefe gives, as an example, the following sentence that contains a similar mistake: “A person who cares only about their own happiness will not achieve happiness.”