of When could have, might have, must have, and similar phrases are spoken, they usually come out as contractions: Could've, might've, must've and so on. Because the contracted form 've sounds like of, some people mistakenly write could of, might of, must of.
Nonstandard: That plant must of been over watered.
Standard: That plant must have been over watered.
past, passed Past is a noun, adjective or preposition ("Adele read the minutes of the past meeting" "The dog walked past the cat"). Passed is a verb, the past tense of pass ("We passed the papers to the front").
personal, personnel Personal means "of a person": "a personal opinion," "a personal matter." Personnel refers to the people in an organization, especially employees: "Administrative personnel will not be affected."
piece, peace Piece is a portion of a larger object ("a piece of pie") and peace means tranquility ("peace and quiet").
principal, principle The basic meaning of principal is "chief" or "most important." It is used in this sense both as a noun and as an adjective: "The principal of a school, " "the principal point." It is also used to refer to a capital sum of money, as contrasted with interest on the money: "He can live on the interest without touching the principal." Principle is used only as a noun and means "rule," "law," or "controlling idea": "the principle of 'one man, one vote'"; "Cheating is against my principles."
proceed, precede To proceed is to "go forward"; to precede means "to go ahead of"; "The blockers preceded the runner as the football team proceeded toward the goal line."
rain, rein, reign Rain means precipitation ("The rain fell gently on the flowers"). Rein means a harness strap ("The horse's reins") or control ("The reins of government"). To reign means to rule or a period of rule ("Queen Elizabeth reigns over England").
refer back A confusion between look back and refer. This usage is objected to in formal writing on the ground that since the re- of refer means "back," refer back is redundant. Refer back is acceptable when it means "refer again" ("The bill was referred back to the committee"); otherwise, use refer ("Let me refer you to page 17").
respectfully, respectively Respectfully means with respect or full of respect ("The reporters listed respectfully to the senator's request"). Respectively means each in the order given ("Nick, Margo, and Ted are nineteen, seventeen, and fifteen, respectively").
so (conj.) The use of "so" as a connective ("The salesperson refused to exchange the merchandise; so we went to the manager") is thoroughly respectable, but its overuse in formal writing is objectionable. There are other good transitional connectives -- accordingly, for that reason, on that account, therefore -- that could be used to relieve the monotony of a series of so's. Occasional use of subordination ("When the salesperson refused to exchange the merchandise, we went to the manager") also brings variety to the style.
stationery, stationary Stationery is writing paper ("Perfumed stationery is in bad taste"), while stationary means in a fixed position ("The desk is stationary").
than, then Than is a conjunction used in comparison; then is an adverb indicating time. Do not confuse the two: "I would rather write in the morning than in the afternoon. My thinking seems to be clearer then."
that, which, who That refers to persons or things, which refers to things, and who refers to persons. That introduces a restrictive clause; which usually introduces a nonrestrictive clause: "John argued that he was not prepared to take the exam. The exam, which had been scheduled for some time, could not be changed." "Anyone who was not ready should have to take the test anyway."
there, their, they're Although these words are pronounced alike, they have different meanings. There indicates place: "Look at that dog over there." Their indicates possession: "I am sure it is their dog." They're is a contraction for "they are": "They're probably not home."
unique The formal meaning of unique is "sole" or "only" or "being the only one of its kind": "Adam was unique in being the only man who never had a mother." The use of "unique" to mean "rare" or "unusual" ("Americans watched their television sets anxiously as astronauts in the early moon landings had the unique experience of walking on the moon") has long been popular, but some people still object to this usage. The use of "unique" to mean merely "uncommon" ("a unique sweater") is generally frowned upon. Unique should not be modified by adverbs that express degree; very, more, most, rather.