Then & Than

Then: An adverb indicating time
"Mother explained, and then I understood what she meant."

Than: A conjunction that is used in comparisons
"I have more books than I expected."

Whose & Who's

Whose: The possessive form of "who"
"Whose watch is this?"

Who’s: The contraction of "who" and "is"
"Who’s on first base?"

Biannual, Biennial, Semiannual

"Biannual" and "semiannual" both mean "twice a year." "Biennial" means "every two years." Because of the possible confusion between "biannual" and "biennial," use "semiannual" when you want to describe something that occurs twice a year.

Aid vs. Aide

Aid (noun): a form of help.
(verb): to help.

Aide: an assistant.

Affect vs. Effect

"Affect" is generally used as a verb meaning to change, assume, or influence. "Effect" is usually a noun meaning impression or result; however, "effect" can also be a verb meaning to bring about.

Examples:
To effect the sale from your account, we require written instructions.
The president´s decision will have a direct effect on the department.
The decision will affect my job.
Establishing this account will not affect your tax liability.

Regard vs. Regards

Use "in regard to," "with regard to," "regarding," or "as regards." Never use "in regards to." (The other acceptable use is "Give my regards to Broadway.")

Anybody vs. Any Body

"Anybody" means "any person." "Any body" means "any corpse," or "any human form," or "any group."

Altogether vs. All Together

He is "altogether" (entirely) too lazy.
The papers are "all together" (all in a group) in the binder.

Adherence vs. Adherents

Adherence: attachment

Adherents: followers

"Obligated" versus "Obliged"

"Obligate," meaning 1. to bind or compel, legally or morally and 2. to commit (money, funds, etc.) to meet an obligation, has connotations of legal and ethical constraints. "Oblige," meaning 1. to constrain by physical, moral, or legal force or by circumstance and 2. to put in one's debt or do a favor or service for, has connotations of constraints more closely associated to protocol, etiquette, and gratitude. It is usually considered more appropriate to use "oblige" to express appreciation, e.g. "They obliged us by not bringing their children" and "John was much obliged for your assistance."

All Right

It is always two words, never "alright."

Adverse vs. Averse

Adverse: hostile, unfavorable

Averse: disinclined

Ensure, Insure, Assure

Ensure: to make certain.
Insure: to protect against loss.
Assure: to give someone confidence. (The object of this verb should always refer to a person.)

Examples:
I want to ensure (make certain) that nothing can go wrong.
I want to assure you (give you confidence) that nothing will go wrong.
I want to insure this necklace (protect it against loss) for $5,000.

Broach vs. Brooch

Broach: to introduce (like a topic)

Brooch: an ornamental pin

Accept vs. Except

Accept: To take, to receive.

Except (verb): To exclude.
(preposition): excluding

Example: I would accept your criticism, except you don´t know what you´re talking about.

Principal vs. Principle

"Principle" is only a noun, and all of its meanings are abstract. "Principal" is both a noun and an adjective. As a noun, it generally refers to a person, except in specialized legal and financial terms.

Ex: Approximately $40 of your payment goes repay the principal balance; the rest are interest payments.
Ex: I have to meet with the principal of my child´s school.
Ex: We have to act on the principle that our customers are important.

Ante- vs. Anti-

Ante-: A prefix meaning "before"

Anti-: A prefex meaning "against"

Air vs. Heir

Air: atmosphere

Heir: one who inherits

Correct & Incorrect Words

afterwards ("afterward" is correct)
come and see me ("come to see me" is correct)
in route ("en route" is correct)
irregardless ("regardless" is correct)
more importantly ("more important" is correct 99% of the time)
secondly, thirdly (use "second," "third," etc.)
towards ("toward" is correct)

Addition vs. Edition

Addition: something added

Edition: One version of a printed work