By now you should be persuaded that speaking correctly is “a good thing,” and you are almost there. Be careful! “They” can be a pitfall.
Two different kinds of “they” can cause trouble. The first one is dealt with on the “Everyone” is Singular” page. (Remember? “Everyone must sit in their chair” is a grammatical error, as “everyone” is singular, and “their” is a plural possessive. Say “his,” “her” or “his or her” instead.)
“They” is a trouble-maker in a second way: It is the use of “they” as an indefinite pronoun. Examples: “They said we didn’t have school tomorrow” and “They call me ‘the Dude.‘” This second problem is an error of rhetoric; it is inarticulate. Why? Who is they?” Remember from grammar that all pronouns must have antecedents, that is, they must take the place of a noun, and the identity of that noun should be clear. That rule describes the problem of an “indefinite antecedent.” In this case, there is no antecedent at all!
If the objectives of good rhetoric are clarity and precision (and they are), the use of “they” with no specific antecedent should be avoided.
My high school chemistry professor used to say, “One day, I’d like to catch up with this ‘they’ fellow, and give him a piece of my mind . . . he goes around spreading rumors that cause trouble for all of us.”
The better practice is to use a real subject for your sentence, as in: “The teacher said we have no school tomorrow” and “People call me ‘the Dude.‘” If you want to avoid revealing who “they” really is, use the passive voice: “I was told that we have no school tomorrow” and “I am called ‘the Dude.’” Another way to say it is: “I understand that we have no school tomorrow” and “Please call me ‘the Dude.‘”
There is always a better way to express yourself than to use “they” with no specific antecedent.
For a somewhat related subject, see this post on the indefinite “you” problem.