a/an: Use “a” before consonant sounds and “an” before vowel sounds:
Example: Lisa wants to be a history teacher.

The award was quite an honor.

After a few more courses, she will have an MBA.

about/around

“about” means approximately; “around” refers to location.

affect/effect

“Affect” is a verb meaning “to influence or put on”: My choices will affect my future. “Effect” is a noun meaning “result”: The effect of my choice is obvious.

a lot

not “alot”

all right

not “alright”

allude/elude

“Allude” means to make an indirect reference to something; “elude” means to evade or escape detection.

alternately/alternatively

“Alternately” means one after the other; “alternatively” means instead of.

beside/besides

“Beside” means “next to” or “at the side of.” “Besides” means “in addition to.”

bi/semi

“Bi” means two, and “semi” means half.

biennial

bimonthly

can/may

“Can” refers to ability; “may” refers to possibility and permission.

continual/continuous

"Continual" means repeated or intermittent; "continuous" means unbroken.

differ from/differ with

“Differ from” means to be unlike; “differ with” means to disagree.

disinterested/uninterested

“Disinterested” means impartial; “uninterested” means to lack interest.

each/either/neither

When used as subjects, these three pronouns always take singular verbs. When they are used as adjectives, the nouns they modify always take a singular verb.

Examples: Each is unique in his or her own way.

Each student gives a presentation at the end of his or her research.

Of the two professors, either will conduct dynamic classes.

Either school offers advanced courses in theological studies.

Neither writes for newspaper anymore.

Neither candidate believes that the school should increase spending.

farther/further

"Farther" expresses physical distance: I can run farther than my sister. "Further" refers to degree, time or quantity: We will discuss this further tomorrow.

in/into

These prepositions are not interchangeable. “In” refers to location or position; “into” indicates motion.

in order to

Too wordy – you can usually omit “in order”: ( She came over in order to show me her new shoes. )

irregardless

lay/lie

“Lay” is a transitive verb and requires a direct object. “Lie” is an intransitive verb and never takes a direct object. " Before I lie down (no direct object), I will lay my book on the table ." (Book is the direct object.)

Be careful of confusing lie and lay in the past tense. The past tense of “lie” is “lay” (lie, lay, lain, lying); the past tense of “lay” is “laid” (lay, laid, laid, laying).

less/fewer

Use “fewer” with a number of individual items and “less” with quantity, amount or bulk: Fewer than 10 graduates had a GPA of less than 3.0.

more than/over

These words are not interchangeable. “Over” refers to a spatial relationship; “more than” refers to figures and amounts.

nonprofit/not-for-profit

These are interchangeable terms.

people/persons

Use “people” instead of “persons” when referring to more than one “person”.

plus

Use only in adding units. Otherwise, use “in addition,” “also” or “and.”

proved/proven

“Proved” is the past participle of the verb prove ( She proved her innocence ). “Proven” is an adjective ( It is a proven fact that females mature faster than males) .

preventive/preventative

“Preventive” is preferred. (The extra syllable is unnecessary.)

semiannual

twice a year

semimonthly

twice a month

set/sit

Both are verbs, but “set” requires an object ( Will you set your package on the table? ) while “sit” never takes an object ( Please sit down after you turn in your paper ).

since/because

These words are not the same. Use “since” to refer to a period of time and “because” to give a reason or cause.

than/then

Use “than” as a conjunction of comparison and “then” as an adverb denoting time.

toward

Not “towards.”

utilize

“Use” or “employ” are preferable: He used his contacts to get into the big event.

United States or U.S.

Spell it out when used as a noun: The United States is at war with Iraq. Only use U.S. (no space) as an adjective: He bought a large U.S. map for his classroom.

upside down / upside-down

“Upside down” is an adverb: The plane flew upside down. “Upside-down” is an adjective: I can’t read the writing because it is upside-down.

up-to-date

hyphenate

versus

Denotes “against” or “in contrast with.” Spell it out in general text but you may abbreviate it to “vs.” in common expressions like “war vs. peace.” Only use “v.” in legal cases (Roe v. Wade).

who, which, that

"That" is used to restrict meaning, and "which" is used to elaborate on it. "Who" can be used for both people or things when they are endowed by the writer with human qualities.

Example: He is wearing the sweater that I bought him for Christmas.

The sweater, which is red and cable-knit, is very stylish.
Elizabeth, who sold me the sweater, knows quite a bit about winter fashion.

who's/whose

“Who’s” is the contraction of “who is”: Who’s coming to the party? “Whose” is a possessive pronoun: Whose house is the party at?