affect/effect: "affect" is most often used as a verb (very rarely as a noun) to mean "influence." "Effect" is almost always used as a noun meaning "result." When used as a verb, "effect" means "to bring about."
all together/altogether: the first means "everyone gathered"; the adverb "altogether" means "wholly" or "entirely."
alternative /alternate: the first means "a choice among several possibilities." By contrast, "alternate" as a verb means "to follow by turns" or "rotate." It comes a bit closer to "alternative" when it is used as a noun to mean a "substitute" or "second choice."
attribute/contribute: these verbs mean two very different things. "Attribute" is "to credit" someone or something for something, or " to ascribe" ; "contribute" means "to make a contribution."
bad/badly: "bad" is an adjective; "badly" is an adverb. "That is a bad idea" vs. "My foot hurt badly after breaking it in the tennis match."
breach/broach: as verbs, these words mean respectively "to make a hole or gap in" and "to begin to talk about."
cite/site: "cite" is a verb that means "to quote or mention as an authority or example," as in, "He cited several sources from the Vatican." The noun "site" refers to a place: "She spends her summers at the archaeological site in North Africa."
compliment/complement: As either nouns or verbs, these words differ in meaning in the following way: the former has to do with flattery ("She complimented him on his manners"), the latter with "completing something": "wearing that blue tie will complement your white shirt."
convince/persuade: the difference here is subtle: you convince someone of something, and persuade someone to take action or do something. "I convinced her that she would enjoy vocal camp, and thus persuaded her to fill out the application."
council/counsel: the noun "council" refers to an assembly gathered for deliberation; used as a noun, "counsel" means "a discussion," though it is more typically used as a verb meaning "to recommend." (Don't confuse either with the Roman office of "consul"!)
criterion/criteria: this word (in singular and plural) brings us to the issue of foreign loan words. Their Greek endings reflect their origin, but confuse English speakers. Just remember that the –on ending is singular (think "one") and the –a ending is plural.
currently/presently: "presently" seems to be fast overtaking "currently," perhaps because it sounds more formal. Technically, however, "presently" means "soon," not "now," though this usage has been increasing so dramatically that everyone except old farts like me has resigned herself to accepting them as interchangeable.
disinterested/uninterested: these two words are not interchangeable. Whereas "uninterested" means that you have no interest in something, "disinterested" means that you have no investment in something. Think of advice columnist Dear Abby as a kind of "disinterested third party" who will give you non-partial advice.
eminent/imminent: of these two adjectives, the first means "prominent" ("our speaker is an eminent physician in town"), the second "impending" or "about to occur" ("the arrival of the storm is imminent").
fewer/less: often confused, these two terms mean the same thing but are used in different circumstances: "fewer" is used when what you are referring to is something that can be counted, such as coins or people (e.g., "there are fewer calories in a rice cake than in a chocolate cake"); "less" is used only when you are referring to something that cannot be enumerated, as in money or clothing ("we'll have less money for our trip if we buy that ceramic vase today").
flout/flaunt: one "flouts" something, such as the law, by scorning it or showing contempt, whereas one "flaunts good looks," for example, by showing off ("If you've got it, flaunt it," the saying goes).
fortunate/fortuitous: everyone knows what it means to be fortunate, but unfortunately too many folks want to use "fortuitous" as if it means the same thing; it doesn't. "Fortuitous" means rather "to happen by accident or chance; unplanned": "It was merely fortuitous [completely by chance] that we ran into each other at the car show."
fulsome/full: all know the meaning of the second word, but often assume the first
word means the same thing, with folks using "fulsome" to mean, incorrectly, "abundant." Actually, "fulsome" means "offensively excessive or insincere; loathsome; disgusting." It would be only rare, then, to wish to speak of "fulsome praise."
good/well: see bad/badly. It's the same situation: "He did a good job on his project," and thus "his project turned out well." Please don't tell those little soccer players running off the field that they "played so good!" when "well" is what you mean.
infer/imply: "Infer" has to do with what you conclude from someone's remarks or actions; "imply" means "to suggest." "He inferred that she was upset with him, even though she thought that her behavior implied she was ready to make up."
lie/lay: it's really simple to distinguish these two verbs if you take the time: the first, a so-called "intransitive" verb, takes no direct object: "I lie down." By contrast, "lay" is a "transitive" verb and thus takes a direct object: "They lay their books on the table." If you memorize the principal parts of these verbs, you'll be set: present/past/perfect tenses = lie/lay/lain and lay/laid/laid. So, next time you speak to your dog, tell him to "lie down," not "lay down"; once he hears it correctly, I'm sure he'll obey! For more on this, click here.
lead/led: the biggest problem with usage here is the incorrect use of "lead" for the past tense. Here's the correct usage: "As of today, I lead the department in broken bones, but until this year, Dr. Baker led the field."
lose/loose: we all know what these mean, but too often I see "loose" when the writer means "lose." I lose a sock from time to time, but my shoelaces rarely come loose.
loath/loathe: the first is an adjective ("I am loath to eat squid"), the second a verb ("Everyone loathes housecleaning").
may/might: technically "may" is present tense and "might" is past tense, but "might" can be used in the present tense to convey something more tentative. "He may come to dinner" assumes it's more likely than "he might come to dinner."
nauseated/nauseous: when you're sick, you're not likely to care about the difference, but there is one. When we say we're "nauseous," we really mean we're "nauseated." How so? Something "nauseous" is like something toxic: it makes you sick ("that disgusting toilet is nauseous"). When the stomach just won't calm down, we're indeed "nauseated."
phenomenon/phenomena: see criterion/criteria above. It's the very same principle. So don't say that that orange and pink streak across the sky was an amazing phenomena! Just one, please.
principal/principle: you learned in grade school that the principal is your "pal," helping you remember that the first word here, used as a noun, refers to the head of a school. It can also be used as an adjective to mean "chief" or "primary," as in"he is the principal investigator for the county." By contrast, "principle" is only a noun referring to fundamental laws or values (or an adjective, "principled.")
proactive/active: there's nothing in the first word that the second word does not fully convey; absent from my dictionary, this word is yet another example of how bureaucratic nonsense—like a virus—invades our language because of its insatiable need to create "new" words, presumably necessary to connote the supposedly oh-so-amazing new concept bureaucrats seem convinced they create. (They're also fond of turning perfectly good nouns into heretofore non-existent verbs—witness "impact"—and verbs into nouns, such as "disconnect," where it should be "disconnection.")
ravage/ravish: "ravage" means to destroy: "The Visigoths ravaged Rome in AD 410. "Ravish" has a rather different meaning: "to seize or carry away by force," usually with the intent to violate sexually. Take extra care to choose the correct word here!
regimen/regime: technically, it appears that these nouns can be used interchangeably, but "regimen" is the preferred term to mean "a system of therapy" or a regimented program of some kind, whereas "regime" better refers to a government or administration.
which/that: O'Connor provides two rules to help us discern which word to use when beginning a clause: 1) "If you can drop the clause and not lose the point of the sentence, use which. If you can't, use that." 2) "A which clause goes inside commas. A that clause doesn't." Here are two examples I've made up: "I've finished this column, which is a relief." "Now I can tackle the second article that goes in the newsletter."