Beware of the Notorious Confusables!

affect / effect
Effect is usually a noun that means a result or the power to produce a result:
-“The sound of the falling rain had a calming effect, nearly putting me to sleep.”
Affect is usually a verb that means to have an influence on:
-“His loud humming was affecting my ability to concentrate.”
Note that effect can also be a verb meaning to bring about or execute:
-“The speaker's somber tone effected a dampening in the general mood of the audience.”

all right / alright
Although alright is widely used, it is considered nonstandard English. As the American Heritage Dictionary notes, it's not “all right to use alright.”

all together / altogether
All together is applied to people or things that are being treated as a group.
-“We put the pots and pans all together on the shelf.”
All together is the form that must be used if the sentence can be reworded so that all and together are separated by other words:
-“We put all the pots and pans together on the shelf.”
Altogether is used to mean entirely:
-“I am altogether pleased to be receiving this award.”

allusion / illusion
Allusion is a noun that means an indirect reference:
-“The speech made allusions to the final report.”
Illusion is a noun that means a misconception:
-“The policy is designed to give an illusion of reform.”

alternately / alternatively
Alternately is an adverb that means in turn; one after the other:
-“We alternately spun the wheel in the game.”
Alternatively is an adverb that means on the other hand; one or the other:
-“You can choose a large bookcase or, alternatively, you can buy two small ones.”

beside / besides
Beside is a preposition that means next to:
-“Stand here beside me.”
Besides is an adverb that means also:
-“Besides, I need to tell you about the new products my company offers.”

bimonthly / semimonthly
Bimonthly is an adjective that means every two months:
-“I brought the cake for the bimonthly office party.”
Bimonthly is also a noun that means a publication issued every two months:
-“The company publishes several popular bimonthlies.”
Semimonthly is an adjective that means happening twice a month:
-“We have semimonthly meetings on the 1st and the 15th.”

capital / capitol
The city or town that is the seat of government is called the capital; the building in which the legislative assembly meets is the capitol. The term capital can also refer to an accumulation of wealth or to a capital letter.

cite / site
Cite is a verb that means to quote as an authority or example:
-“I cited several eminent scholars in my study of water resources.”
It also means to recognize formally:
-“The public official was cited for service to the city.”
It can also mean to summon before a court of law:
-“Last year the company was cited for pollution violations.”
Site is a noun meaning location:
-“They chose a new site for the factory just outside town.”

complement / compliment
Complement is a noun or verb that means something that completes or makes up a whole:
-“The red sweater is a perfect complement to the outfit.”
Compliment is a noun or verb that means an expression of praise or admiration:
-“I received compliments about my new outfit.”

comprise / compose
According to the traditional rule, the whole comprises the parts, and the parts compose the whole. Thus, the board comprises five members, whereas five members compose (or make up) the board. It is also correct to say that the board is composed (not comprised) of five members.

concurrent / consecutive
Concurrent is an adjective that means simultaneous or happening at the same time as something else:
-“The concurrent strikes of several unions crippled the economy.”
Consecutive means successive or following one after the other:
-“The union called three consecutive strikes in one year.”

connote / denote
Connote is a verb that means to imply or suggest:
-“The word ‘espionage’ connotes mystery and intrigue.”
Denote is a verb that means to indicate or refer to specifically:
-“The symbol for ‘pi’ denotes the number 3.14159.”

convince / persuade
Strictly speaking, one convinces a person that something is true but persuades a person to do something.
-“Pointing out that I was overworked, my friends persuaded [not convinced] me to take a vacation. Now that I'm relaxing on the beach with my book, I am convinced [not persuaded] that they were right.” Following this rule, convince should not be used with an infinitive.

council / councilor / counsel / counselor
A councilor is a member of a council, which is an assembly called together for discussion or deliberation.
A counselor is one who gives counsel, which is advice or guidance. More specifically, a counselor can be an attorney or a supervisor at camp.

discreet / discrete
Discreet is an adjective that means prudent, circumspect, or modest:
-“Her discreet handling of the touchy situation put him at ease.”
Discrete is an adjective that means separate or individually distinct:
-“Each company in the conglomerate operates as a discrete entity.”

disinterested / uninterested
Disinterested is an adjective that means unbiased or impartial:
-“We appealed to the disinterested mediator to facilitate the negotiations.”
Uninterested is an adjective that means not interested or indifferent:
-“They seemed uninterested in our offer.”

elicit / illicit
Elicit is a verb that means to draw out. Illicit is an adjective meaning unlawful. “No matter how hard I tried to elicit a few scandalous stories from her, she kept all knowledge of illicit goings-on discreetly to herself. ”

emigrant / immigrant
Emigrant is a noun that means one who leaves one's native country to settle in another:
-“The emigrants spent four weeks aboard ship before landing in Los Angeles.”
Immigrant is a noun that means one who enters and settles in a new country:
-“Most of the immigrants easily found jobs.”
One emigrates from a place; one immigrates to another.

farther / further
Farther is an adjective and adverb that means to or at a more distant point:
-“We drove 50 miles today; tomorrow, we will travel 100 miles farther.”
Further is an adjective and adverb that means to or at a greater extent or degree:
-“We won't be able to suggest a solution until we are further along in our evaluation of the problem.”
It can also mean in addition or moreover:
-“They stated further that they would not change the policy.”

few / less
Few is an adjective that means small in number. It is used with countable objects:
-“This department has few employees.”
Less is an adjective that means small in amount or degree. It is used with objects of indivisible mass:
-“Which jar holds less water?”

figuratively / literally
Figuratively is an adverb that means metaphorically or symbolically:
-“Happening upon the shadowy figure, they figuratively jumped out of their shoes.”
Literally is an adverb that means actually:
-“I'm not exaggerating when I say I literally fell off my chair.”
It also means according to the exact meaning of the words:
-“I translated the Latin passage literally.”

flammable / inflammable
These two words are actually synonyms, both meaning easily set on fire. The highly flammable (inflammable) fuel was stored safely in a specially built tank. Use nonflammable to mean not flammable.

flaunt / flout
To flaunt means to show off shamelessly:
-“Eager to flaunt her knowledge of a wide range of topics, Helene dreamed of appearing on a TV trivia show.” To flout means to show scorn or contempt for:
-“Lewis disliked boarding school and took every opportunity to flout the house rules.”

foreword / forward
Foreword is a noun that means an introductory note or preface:
-“In my foreword I explained my reasons for writing the book.”
Forward is an adjective or adverb that means toward the front:
-“I sat in the forward section of the bus.”
-“Please step forward when your name is called.”
Forward is also a verb that means to send on:
-“Forward the letter to the customer's new address.”

founder / flounder
In its primary sense founder means to sink below the surface of the water:
-“The ship foundered after colliding with an iceberg.”
By extension, founder means to fail utterly.
Flounder means to move about clumsily, or to act with confusion. A good synonym for flounder is blunder:
-“After floundering through the first half of the course, Amy finally passed with the help of a tutor.”

hanged / hung
Hanged is the past tense and past participle of hang when the meaning is to execute by suspending by the neck:
-“They hanged the prisoner for treason.”
-“The convicted killer was hanged at dawn.”
Hung is the past tense and participle of hang when the meaning is to suspend from above with no support from below:
-“I hung the painting on the wall.”
-"The painting was hung at a crooked angle.”

historic / historical
In general usage, historic refers to what is important in history, while historical applies more broadly to whatever existed in the past whether it was important or not:
-“a historic summit meeting between the prime ministers;”
-“historical buildings torn down in the redevelopment.”

i.e. / e.g.
The abbreviation e.g. means for example (from Latin exempli gratia):
-“Her talents were legion and varied (e.g., deep sea diving, speed reading, bridge, and tango dancing).”
The abbreviation i.e. means that is or in other words (from Latin id est):
-“The joy of my existence (i.e., my stamp collection) imbues my life with meaning.”

it's / its
It's is a contraction for it is, whereas its is the possessive form of it:
-“It's a shame that we cannot talk about its size.”

laid / lain / lay
Laid is the past tense and the past participle of the verb lay and not the past tense of lie. Lay is the past tense of the verb lie and lain is the past participle:
-“He laid his books down and lay down on the couch, where he has lain for an hour.”

lend / loan
Although some people feel loan should only be used as a noun, lend and loan are both acceptable as verbs in standard English:
-“Can you lend (loan) me a dollar?”
However, only lend should be used in figurative senses:
-“Will you lend me a hand?”

lightening / lightning
Lightening is a verb that means to illuminate; lightning is a noun referring to the electrical charges the cause flashes of light during storms:
-“The lightning struck, lightening the sky.”

nonplussed
Meaning perplexed or bewildered, nonplussed is very often thought to mean just the opposite—calm, unruffled, cool-as-a-cucumber. A common mistake is to think the word means not “plussed,” but no such word exists. Nonplussed originates from the Latin non (no) and plus (more, further), and means a state in which no more can be done—one is so perplexed that further action is impossible. “The lexicographer grew increasingly agitated and nonplussed by the frequency with which she noted the misuse of nonplussed.”

passed / past
Passed is the past tense and past participle of pass. Past refers to time gone by; it is also a preposition meaning beyond.
-“In the past decade, I passed over countless opportunities; I was determined not to let them get past me again.”

penultimate
Meaning “next to last,” penultimate is often mistakenly used to mean “the very last,” or the ultimate:
-"The perfectionist was crestfallen when he was awarded the penultimate prize; the grand prize went to another.”

precede / proceed
The verb precede means to come before. Proceed means to move forward.
-“He preceded me into the room; once I caught up with him I proceeded to tell him off.”

principal / principle
Principal is a noun that means a person who holds a high position or plays an important role:
-“The school principal has 20 years of teaching experience.”
Principal is also an adjective that means chief or leading:
-“The necessity of moving to another city was the principal reason I turned down the job offer.” Principle is a noun that means a rule or standard:
-“They refused to compromise their principles.”

stationary / stationery
Stationary is an adjective that means fixed or unmoving:
-“They maneuvered around the stationary barrier in the road.”
Stationery is a noun that means writing materials:
-“We printed the letters on company stationery.”

their / there / they're
Their is the possessive form of they; there refers to place; and they're is the contraction of they are.
-“They're going there because their mother insisted they become proficient in Serbo-Croatian.”

venal / venial
Venal is an adjective that means corruptible; venial is an adjective that means a slight flaw or offense:
-“In the Catholic church, a venial sin is one that is minor and pardonable, whereas a mortal sin is a serious transgression involving more venal or depraved behavior.”

who's / whose
Who's is the contraction of who is. Whose is the possessive form of who.
-“Who's going to figure out whose job it is to clean the stables?”

your / you're
Your is the possessive form of you; you're is the contraction you are.
-“If you're planning on swimming, then be sure to bring your life vest and flippers.”