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.
Face lift, face-lift, facelift: All three forms are common, but the one-word form will win in the end.
Facetious vs. fatuous: Facetious: jocular or sarcastic, often but not necessarily in a mocking way. Fatuous: smugly foolish.
Faeces vs. feces: feces in North America; faeces everywhere else.
Faint vs. feint: A feint is a fake attack meant to draw defensive action. The word doubles as a verb meaning to make a feint. Faint, of course, has many meanings, including (1) to pass out, (2) lacking strength, (3) dim, and (4) an abrupt, usually brief loss of consciousness.
Family terms, capitalization: Words like father, mother, sister, and brother are capitalized when they are treated as names, and uncapitalized when they are treated as common nouns. The rule of thumb is that if there is no pronoun or article preceding it, it is capitalized (e.g., Go ask Dad.), and if there is a pronoun or article preceding it, it is uncapitalized (e.g., Go ask your dad.)
Fancy oneself: to believe or imagine oneself to be (something). It’s often facetious.
Far away vs. faraway: Make it two words if it comes after what’s far away (e.g., those places are far away). Make it one word if it comes before what’s far away (e.g., those faraway places). The one-word form is sometimes hyphenated—far-away.
Feckless: Feck is indeed a word, but it’s rare and mostly archaic. It’s related to effect, so we might think of feckless as meaning without effect.
Feet vs. foots: Feet is the plural of foot. Foots is a simple-present inflection of the verb foot, meaning to pay (especially a bill).
Fell: As a verb, it means (1) to cause to fall by striking, or (2) to kill. These senses are most often used participially, in phrases such as the felled tree and the felled warrior. As an adjective, fell means inhumanly sinister or fierce.
Ferment vs. foment: To ferment is (1) to use yeast to convert sugar to carbon dioxide and alcohol, and (2) to make turbulent or bring to a state of unrest. To foment is to promote the growth of.
Fete: (1) a party or festival, (2) to celebrate with a part or festival, or (3) to honor (someone). To be feted is to be honored. The French word from which fete is derived has a circumflex over the first e—to make fête—but the mark is usually dropped in English.
Fiat: Something done by fiat is done by order of an authority. The word sometimes connotes arbitrariness.
Fie: archaic interjection expressing disapproval.
Finagle: to obtain by sneaky or deceptive methods.
Financial vs. fiscal: Both mean of or relating to finances, but fiscal is usually used in reference to government finances.
Finite verb: a verb that serves as the predicate of a clause—e.g., She went. He sneezed.
Fir vs. fur: Fir is a kind of tree. Fur is a coat of hair.
Firebrand: someone who energetically stirs up trouble.
Firsthand, secondhand, thirdhand: These are unhyphenated. Fourth-hand, fifth-hand, etc. are hyphenated.
Flat adverbs: adjectives that double as adverbs—i.e., without the -ly suffix . E.g., The sun shone bright. I will come quick. The old woman drives slow. Most flat adverbs have corresponding -ly forms that mean the same. Some strict English users believe the flat forms are incorrect or inferior.
Flip side vs. flipside: The two-word form prevails, but the one-word form is gaining ground.
Fly in the face of: to openly flaunt or oppose.
Fly off the handle: to become enraged, especially suddenly. The phrase connotes irrationality. One who flies off the handle isn’t thinking clearly and is likely to regret it later.
Focus: makes focused and focusing—one s—not focussed and focussing. This is the case in all main varieties of English.
Foist: (1) to pass off as genuine; (2) to impose by force or trickery.
Folderol: foolishness or nonsense.
Foot long, footlong, foot-long: It’s one word as an adjective preceding what it modifies (e.g., a footlong sub) and as a noun (I ordered three footlongs). It’s two words as an adjective phrase coming after what it describes (the sub is a foot long). The hyphenated form is a variant of foot-long and is likely to diminish over time.
For ever vs. forever: Some style guides make the distinction that forever means continually and for ever means eternally, but the distinction is rarely borne out in practice, and it’s safe to use forever for both uses.
For the birds: worthless.
Forceful vs. forcible: no significant difference.
Formulae vs. formulas: Formula is an English word when English-speakers use it, and this has been so for centuries, so we can pluralize it in the English manner—formulas. But like many other Latin plurals, formulae prevails in mathematics and science.
French fries: There is no need to capitalize the french in french fry.
Fringe benefits: benefits that are supplemental to the main benefits. Not French benefits.
From time to time: no hyphens needed.
Front line vs. frontline: two words, unless you’re talking about the PBS television show.
Frown upon: disapprove of.