It's vs. its, who's vs. whose, and you're vs. your
Students often use 'they' and 'their' as generic singular pronouns because they want to avoid using 'he' and 'his,' which are male pronouns, to refer to people generally. There are other ways to avoid this, however.
Note: 'they' and their' are often used as generic singular pronouns in conversation, and some people have claimed that this usage was common even in writing at some points in the past. I believe that usage determines 'correct' grammar, and at some point in the future, the 'generic singular' they and their might be generally considered acceptable. However, standards in written language (especially academic written language) tend to be less relaxed than those in spoken English, and also to lag behind spoken English.
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