Note: Many of the entries here will eventually become full-length posts. Some are rough and have not been fully researched. If you have any corrections or would like to add anything, please comment.
(The) dark continent: an antiquated, now offensive term for Africa. Don’t use it.
Day care vs. daycare: The two-word form prevails in American English. Both are common in other varieties of English.
Debark vs. disembark: Disembark: (1) to leave a craft; (2) to take ashore. Debark: (1) to unload; (2) to disembark.
Deer vs. deers: Both are accepted plurals, but deer is far more common and is the safer choice.
Defraud: to commit fraud.
Degrees: bachelor’s, master’s, etc.—with an apostrophe.
Demagogue: a leader who gains power by making emotional appeals that take advantage of listener’s prejudices. This sort of activity is demagoguery.
Demeanor vs. demeanour: demeanor in the U.S.; demeanour everywhere else.
Demob: short for demobilize.
Deplane: to get off the plane. The word is objectionable to many for its illogical construction (none of the definitions of the prefix de- mean get off or leave). Unless you work for an airline or get a kick out of setting peevish readers’ teeth on edge, consider avoiding it.
Dervish: one with abundant, frenzied energy.
Deviled vs. devilled (eggs): deviled in the U.S. and Canada; devilled outside North America.
Diagram vs. diagramme: Diagram is preferred in all varieties of English.
Diaspora: the body of a people dispersed outside their homeland.
Dietician vs. dietitian: Dietitian is preferred everywhere. Dietician is almost completely absent from North American publications, while outside North America it appears about once for every three instances of dietitian. Other than the spelling, there is no difference between them.
Diktat: Think of it as an especially harsh or extreme dictate, where dictate means a command. Also, diktat alone bears the sense a harsh settlement imposed on a defeated party.
Disabuse: to free from a lie or misconception.
Disgruntle: to make resentful or ill-tempered. There is no gruntle in modern English.
Disparateness vs. disparity: There is no difference between them. Disparity is preferred by a huge margin.
Dissatisfied vs. unsatisfied: Dissatisfied: unhappy with what one has received. Unsatisfied: lacking satisfaction.
Ditto: the same as stated or written above or before.
Do right by: to act toward (someone) in an ethical, moral, or honorable way, especially to make up for past wrongs.
(That) does not compute: humorous computerese for I don’t understand or that’s illogical.
Dogged: Two different words with different pronunciations: (1) Dogged (rhymes with logged): past tense of the somewhat rare verb dog, meaning to bother or hinder. (2) Dogged (pronounced as two syllables—dog-ged): tenacious, obstinate, or unyieldingly persistent.
Dominant vs. predominant: synonymous in the sense having greatest influence or importance. Only predominant means most common or conspicuous (without connoting control or influence over others). Only dominant means exercising control or great influence over others.
Dos and don’ts: Spell it like that. No extra apostrophes are needed when making do and don’t plural, but the normal apostrophe in don’t is needed, so dos and donts is wrong.
Dotage: feebleness, especially resulting from old age.
Double down: to double one’s wager or, idiomatically, to double one’s risk or investment.
Double whammy: two unfortunate events happening simultaneously or nearly simultaneously.
Downplay vs. play down: They mean the same. Play down is two words.
Downright: utterly. One word.
Downtime: one word.
Draconian: very severe, strict, or harsh. From the Athenian lawmaker Draco, whose laws were harsh.
Dressed to the nines: flamboyantly dressed.
Drink -> drank -> drunk: Drank is the past tense (I drank a beer yesterday.). Drunk is the participle (I have drunk two beers. Those two beers have been drunk.).
Drive to distraction: (1) to make someone very distracted, (2) to make someone very bored, (3) to confuse someone.
Drop off, dropoff, drop-off: Drop off is a verb—e.g., “I need to drop off the kids.” Drop-off is a noun—e.g., “There was a drop-off in sales last month.” The noun is sometimes spelled without a hyphen—dropoff. This is not yet the prevalent form (and your spell check probably says it’s wrong), but it is gaining ground and is likely to gain widespread acceptance sooner or later.
(In) due course: when the time is right; in the appropriate order.
Due to: not do to.