1. Erroneous use of the word "they"
"They" is plural. It must refer to more than one person. Yet, I often encounter students writing things such as "When someone does philosophy, they grapple with very difficult issues." or "Everyone should believe what they think is most rational." These are ungrammatical. In these contexts, "she or he" ("he or she", etc.) must be used.
2. there / they’re / their
The word "there" is the counterpart to "here". It can function as a noun or as an adverb. Correct uses of "there": "There will be a test tomorrow." "I was sitting there."
The contraction "they're" is shorthand for "they are". It can only be used when "they are" can be substituted. Correct uses: "They're my best friends." "Let's do what they're doing."
The word "their" is the possessive adjective form of "they", just as "my" is the possessive adjective form of "I". Correct uses: "Their test is tomorrow." "They brought their kids."
3. it's / its and who's / whose
The word "it's" is a contraction for "it is". It can only be used where "it is" can be replaced for it. Similarly, "who's" is a contraction for "who is" or "who has". It can only be used where one of these can be substituted. Correct uses: "It's Tuesday." "Who's your favorite TA?"
The word "its" is the possessive of "it". The word "whose" is the possessive of "who". Correct uses: "The door was broken off its hinge." "Whose book did you borrow?"
4. Using "could of", "should of" or "would of"
These are always ungrammatical. The correct forms to use in such cases are "could have", "should have" and "would have", respectively.
5. Other Trouble Spots
Find out and remember the difference between the following. (Ask me if you're unsure.)
accept (v.) / except (prep.)
affect (v.) / effect (n.)
complement (v. / n.) / compliment (v. / n.)
conscience (n.) / conscientious (adj.) / conscious (adj.) / consciousness (n.)
imply (v.) / infer (v.)
purpose (n.) / propose (v.)
than (conj.) / then (adv. / pn. / conj.)
therefore (adv., conj.) / therefor (prepositional adverb)
threw (v.) / through (prep.) / thorough (adj.) / though (conj. / adv.)
your (possessive adj.) / you're (contraction)
who (subject pronoun) / whom (object pronoun)
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