factitious/fictitious/fictional: factitious means contrived (may be real); fictitious is counterfeit; fictional describes a work of imagination
farther/further:farther is used with physical distance; further is most commonly used figuratively for extent or degree
fatalism/determinism:fatalism is the attitude that it is useless to avoid events, with existence managed by an all-powerful external force; determinism is the view that “cause and effect” deeply influences free will and perhaps turning such freedom into delusion
faze/phase:faze is to disturb or embarass; phase describes a stage or aspect
feather bedding/featherbedding:feather bedding is material stuffed into a matress featherbedding is requiring more workers than needed to do a job
fewer/less:fewer is used for things counted [She had fewer pennies in her purse]; less is used for quanity (mass nouns) {She had less money in her purse). Correct useage of these words is crucial in being considered a skilled user of USA English
figuratively/literally: figuratively is not in an exact sense, but in an analogous sense; literally means in an exact sense
fiscal/monetary:fiscal is about bugetary matters; monetary applies to monetary supply
flack/flak: to flack is a slang term for publicist; flak is a type of antiaircraft fire: figuratively a barrage of criticism
flail/flay: to flail is to strike or beat; to flay is to strip the skin from
flair/flare:flair is a distinctive talent; flare is a sudden, unstable flash of light
flammable/inflammable: flammable is preferred to inflammable (which has ambiguous meaning for some)
flaunt/flout: to flaunt is to make a pretentious or crude display; to flout is to contemptously reject, to sneer or jeer
flier/flyer:flier is an aviator and the preferred term for a handbill; Flyer is the proper name for some buses and trains
flotsam/jetsam:flotsam is floating wreckage; jetsam is stuff thrown off a boat that has washed ashore
flounder/founder: a flounder is a fish; to flounder is to flop around or move clumsily; to founder is to sink or become disabled or to bog down
-fold: a as suffix means "multiplied by": twofold means times 2. Unfortunately, many knowledgeable science and technical author writers use -fold to mean "plus"; because of the high risk of misunderstanding, -fold as a suffix should be avoided
following/after: following is best reserved for the causitive actions “subsequent to” (headaches followed medication ingestion) and is rarely a better choice than “after”; after is typically the best word
forbear/forebear: to forbear is to shun or avoid ; a forebear is an ancestor
foreword/preface: foreword (not to be confused with “forward”) is a book’s introduction written by a person other than the book’s author; preface is an introduction to a book written by the book’s author
fortuitous/fortunate: fortuitous describes something planned or happening by chance; fortunate is something good marked by good luck
fragment/fragmentary: fragment describes a piece or pieces broken from the whole ; fragmentary describes disconnected and incomplete portions
fulsome:fulsome means objectionably excessive; not lavish or profuse