Ability, Capacity. Ability means the power to do something, mental or physical ("ability to manage an office"). Capability is the ability to hold, contain or absorb ("a suitcase filled to capacity").
Affect, Effect. Affect as a verb, means "to assume" or "to influence". ("Her voice affected me strangely"). Effect, as a verb means "to cause"; as a noun, it means "result". ("Being captain effected a change in his attitude"). "The decision had a profound effect on labor relations").
Among, Between. The former indicates a relationship of more than two objects; 'between' refers to only two, or to more than two when each object is considered in relation to the others. ("The land was divided among seven farmers". "The rivalry between Joe and Kate is intense". "Trade between nations is desirable".
Complement, Compliment. Complement means something which completes. ("This hat will complement your wardrobe"). A compliment is a flattering comment. ("When she remarked that he was awesome, Ted thanked her for the compliment").
Continual, Continuous. In some uses, these words are synonymous. A subtle distinction is that 'continual' implies 'a closed recurrence in time', in 'rapid succession'; continuous implies, "without interruption". ("I objected most of my sisters' continual quarreling"). "The continuous dripping of water from that leaky faucet unnerved me").
Council, Counsel. Council means 'an assembly', 'a group'. ("A council of labor leaders voted to the proposal"). Counsel is both a noun and a verb ans means 'advice' or 'to advice'. (" 'Work hard and save your money' is indeed wise counsel"). "Our minister will counsel me whenever problems arise").
Emigrate, Immigrate. The former means to 'to leave'; the latter means 'to enter'. ("The foreman emigrated from Russia in 1918"). "In the future, a great number of people from Latin countries will immigrate to Canada").
Envelop, Envelope. The verb en-vel' op (accent on second syllable) means 'to cover', 'to wrap'. ("Fog will soon envelop the island"). En'vel-ope (accent on first syllable) is noun meaning 'a covering'. ("Your invoice is contained in this envelope").
Farther, Further. These are interchangeable; however, some writers prefer farther to indicate 'space', 'a measurable distance', and further to indicate 'greater in quantity, degree, or time' and also 'moreover', 'in addition to'. ("I drove eight miles farther". "Let us give the matter a further consideration").
Formally, Formerly. The first word means 'in a formal manner', 'precisely', 'ceremonially'. The latter means 'in the past'. ("The artist was formally introduced to the President"). ("I was formerly treasurer of my class").
Fort, Forte. Fort means an 'enclosed place', a 'fortified building'. Forte means 'special accomplishment or ability'. ("The old fort has been turned into a museum". "I am not a golfer; tennis is my forte").
Gourmand, Gourmet. These words have to do with eating, but they are different meaning. A gourmand in a large eater. ("Teen-age boys are notorious gourmands'). A gourmet is a fastidious eater, an epicure, 'a person who knows a lot about food and cooking, and who enjoys eating high-quality food'. ("Les Chevalier du Tastevin is an association of well-known gourmets").
Healthful, Healthy. These words are often used interchangeably, but healthful precisely means 'conductive to health'; healthy means 'possessing health'. In other words, place and food are healthful, people and animal are healthy. ("An athlete must be a healthy person because of his healthful daily activities").
Human, Humane. The term human refers to a person. Some particularly precise writers and speakers do not use the word alone to refer to man as man; they say or write 'human being'. However, use the word alone as a noun has a long and respectable background. Humane means 'tender', 'merciful', 'considerate'. ("The general insisted upon humane treatment of all prisoners").
Impractical, Impracticable. Distinctions in the meanings of these words have broken down somewhat, but the former means 'speculative' or 'theoretical'. Impracticable means 'not capable of being used', 'unmanageable'. ("His plan is impractical and his instructions are impracticable").
Jealous, Zealous. The former means 'resentful' or 'envious'; idiom decrees that jealous should be followed by of, not for. ("Ingrid is jealous for Margaret's ring"). Zealous means 'diligent', 'devoted'. ("They were zealous workers on behalf of their candidate").
Later, Latter. The spelling of these words is often confused. They also have different meanings. Later refers to time. ("The train arrived five minutes later than usual"). Latters means 'near or towards the end of something'. ("Building of the new library should begin in the latter part of next year.").
Least, Lest. The former means 'smallest', 'slighest'. The latter means 'for fear that'. ("It was the very least I could do"). ("Close the door lest our secret be overheard").
Leave, Let. Both words are common in several idiomatic expressions implying permission, but Let is standard whearas Leave is not. ("Let [not Leave] me help you wash the dishes").
Loan, Lend. Many careful writers and speakers use 'Loan' only as a noun ("to make a loan") and 'Lend' as a verb ("to lend money"). Because of constant and widespread usage, loan is now considered a legitimate verb to be avoided only in strict formal English.
Luxuriant, Luxurious. 'Luxuriant' refers to abundant growth; 'Luxurious' pertains to luxury. ("The blooms in her garden were luxuriant"). ("Silk curtains gave the simple room a luxurious touch").
Maybe, May be. The first means 'perhaps'. ("Maybe I will go bowling today"). May be (two words) is used to express possibility. ("There may be some food in the refrigerator").
Most, Almost. Most is the superlative of many and much and means 'greatest in amount, quality or degree'. Almost indicates 'very nearly', 'but all'. Most is colloquial when used for Almost. ("He has almost [not most] come to a decision").
Passed, Past. The first word is the past tense of the verb to pass; in its use as a verb, the latter is the past participle. ("He passed quietly by the open doorway". "The years of struggle are now past"). 'Past' is not only a verb; it is also a noun. In one or the other of these two categories, it appears in many expressions which are either colloquial or slangy, among them pass the buck, make a pass at, a pretty pass, pass up, pass out.
Regard, Regards. The latter is used with as to mean 'consider' or 'think'. ("He regards me as a brother"). 'In regard to' and 'with regard to' are idiomatically sound, but both phrases are wordy and jargonistic. In these same phrases, regards is nonstandard. Limit your use of regards to the plural form of the noun regard and singular form of the verb. ("Please give your aunt my regards").
Reference:
Harry Shaw, "Handbook of English" - Second Canadian Edition, McGraw Hill Publications, 1970. [258/59a]
