1. a while / awhile: “A while” is a noun phrase; awhile is an adverb.
2. all together / altogether: All together now — “We will refrain from using that two-word phrase to end sentences like this one altogether.”
3. amend / emend: To amend is to change; to emend is to correct.
4. amount / number: Amount refers to a mass (“The amount saved is considerable&rdquo ;); number refers to a quantity (“The number of dollars saved is considerable&rdquo ;).
5. between / among: The distinction is not whether you refer to two people or things or to three or more; it’s whether you refer to one thing and another or to a collective or undefined number — “Walk among the trees, ” but “Walk between two trees.”
6. biannual / biennial: Biannual means twice a year; biennial means once every two years.
7. bring / take: If it’s coming toward you, it’s being brought. If it’s headed away from you, it’s being taken.
8. compare to / compare with: “Comparing to” implies similarity alone; “compare with” implies contrast as well.
9. compliment / complement: To compliment is to praise; to complement is to complete.
10. comprise, consist of / compose, constitute: Comprise means “include, ” so test by replacement — “is included of” is nonsense, and so is “is comprised of.” The whole comprises the parts or consists of the parts, but the parts compose or constitute the whole.
11. connote / denote: To connote is to convey (“Air quotes connote skepticism or irony&rdquo ;); to denote is to specify (“A stop sign denotes the requirement to halt&rdquo ;).
12. continual / continuous: Continual events are frequently repeated, or intermittent. Continuous events are uninterrupted, or constant.
13. credible / credulous: To be credible is to be authoritative; to be credulous is to be gullible.
14. deserts / desserts: If you eat only cake, pie, ice cream, and the like, you eat just desserts. If you have it coming to you, you get your just deserts as well. (However, the connotation is negative, so hit the gym.)
15. different from / different than: The former phrase is preferred in formal writing; but “differently than” is always correct usage.
16. discreet / discrete :Discreet means “subtle”; discrete means “separate.” (“He discreetly reminded them of their discrete meanings.&rdquo ;)
17. each other / one another: “One another” is preferred in formal writing when more than two of something are being discussed.
18. economic / economical: Economic refers to the science of economics; economical suggests frugality.
19. elemental/elementary: What’s elemental is essential or integral to nature; what’s elementary is basic.
20. ensure / insure / assure: To ensure is to guarantee, to insure is to indemnify, and to assure is to comfort or convince.
21. epidemic / endemic / pandemic: An epidemic is the outbreak of disease in a limited place and time; an endemic disease is a recurring one peculiar to a place or population; a pandemic is pervasive over a wide geographical area.
22. forgo / forego: To forgo is to go without; to forego is to go before (and is generally used only in the forms foregoing and foregone, which are themselves rare).
23. gibe / jibe / jive: To gibe (soft g, as in gym) is to taunt or insult (though jibe is an alternate spelling), to jibe with is to coincide or fit, to jive is to deceive.
24. historic / historical: Something historic is remarkable for its impact on history; something historical is simply an event in history.
25. home in / hone in: To home in is to close in; to hone in is to confuse one word for another. (“Hone in” has no meaning.)
26. jealousy / envy: Jealousy is resentment; envy is covetousness.
27. lay / lie: Lay is transitive, associated with a direct object — “Lay that pencil down.” “Yesterday, I laid that pencil down.” “That pencil has been laid down.” Lie is intransitive, not so associated — “Lie down.” “Last night, I lay down.” “It was my plan to have lain down already.
28. leach / leech: To leach is to dissolve by percolation; to leech is to remove blood with a leech or to exhaust; as a noun, it means a parasitic worm or the human figurative equivalent, or the edge of a sail (also spelled leach).
29. libel / slander: Libel is written defamation; slander is the spoken equivalent.
30. may / might: May refers to factual or possible; might is appropriate for the hypothetical or counterfactual.
31. nauseous / nauseated: To be nauseous is to cause sickness. To be nauseated is to feel sick.
32. notable / noticeable / noteworthy: Something notable is worthy of note. Something noticeable is capable of being noticed. Noteworthy is a synonym of notable, though the former implies the unusual and the latter the commendable.
33. partly / partially: Partly means “in part”; partially means “incomplete” or, rarely, is an antonym for unfairly.
34. peak / pique: To peak is to reach the pinnacle; to pique is to arouse interest or to bother.
35. people / persons: People has assumed primacy; persons is reserved mostly as a synonym for bodies (“those belongings carried on their persons&rdquo ;).
36. persuade / convince: To persuade someone is to motivate them to do something; to convince someone is to lead them to understand or believe.
37. predominantly / predominately: Both forms are correct, but predominantly predominates.
38. purposely / purposefully: What’s done purposely is done on purpose; what’s done purposefully is done with a purpose.
39. regrettably / regretfully: Regrettably is a synonym for unfortunately; regretfully means just that — full of regret.
40. repetitive / repetitious: Both terms have acquired a negative connotation, but the former retains a more neutral meaning.
41. sensual / sensuous: Sensual has an erotic connotation; sensuous refers more neutrally to what is pleasurable to the senses.
42. since / because: Informally, these terms are interchangeable, but in formal writing, since should be used only to refer to time.
43. stationary / stationery: To be stationary is to stand still; stationery refers to letter-writing materials.
44. that / which: That is used restrictively (“The pencil that is sharp” — among more than one pencil, the one with that characteristic); which is employed nonrestrictively (“The pencil, which is sharp” — one pencil alone, possessing that characteristic). The distinction is rarely observed other than in American English.
45. tortuous / torturous: A tortuous experience is a winding one; a torturous one is painful.
46. transcript / transcription: A transcript is a thing; a transcription is the process of creating it.
47. verbal / oral: Verbal refers to both written and spoken communication, but oral is useful for distinguishing the latter from the former.
48. while / although / whereas: Informally, while is a synonym for the other two terms, but in formal writing it should be reserved for temporal connotations.
49. wreak / wreck: These terms do not share etymological origin; you wreck a party, but you do so by wreaking havoc.
50. whether / if: Both words are correct in expressing a choice, but the former is more appropriate in formal writing (“I can’t decide whether to go&rdquo ;), whereas the latter is better reserved for reference to possibility or probability (“I’ll go if you do&rdquo ;).