A, AN Use an before a vowel sound; use a before a consonant sound.
Examples: an apple, a pear, an hour, a horse

ADDITIONALLY Don’t use additionally to substitute for in addition.
Original: Additionally, we would like to see your January report.

Revision: In addition, we would like to see your January report.

AFFECT, EFFECT Effect can be either a noun or a verb; affect can only be a verb (except in the specialized jargon of psychology). As a verb, effect means "to bring about." As a verb affect means "to have an impact on."

Examples: The effect of his decision was far reaching. (noun)

His decision effected vast changes in the company. (verb)

His decision affected every employee in the company. (verb)

ALL RIGHT, ALRIGHT Alright does not exist as a word; the correct term is all right.

ALMOST, MOST Do not use most as a substitute for almost.

Original: I eat ice cream most every day for dessert.

Revision A: I eat ice cream almost every day for dessert.

Revision B: Most days, I eat ice cream for dessert.

AMOUNT, NUMBER Use amount with things thought of in bulk (that you can’t count), and use number with things thought of as individual items (that you can count).

Examples: The number of mistakes in the report was astonishing.

The amount of homework in that course is enormous.

CRITERIA, CRITERION Remember that criteria is plural; the singular form of the word is criterion.

FEWER, LESS Be sure to use the correct one of these two words. Fewer refers to items you can count, like bricks; less refers to mass amounts (things you can’t count).

Examples: This line is for people buying fewer than ten items. 
We had less rain than usual last year.

FIRST, FIRSTLY Firstly – like secondly, thirdly, lastly, etc. – is an unnecessary attempt to add the "ly" form to an adverb. First, last, second, etc. can be adverbs in their own right.

Original: Firstly, I’ll explain the rules. Secondly, we’ll try the exercise.

Revision: First, I’ll explain the rules. Second, we’ll try the exercise.

HOPEFULLY Do not use hopefully as a sort of whole-clause modifier. When you do, hopefully is a dangling modifier.

Original: Hopefully, the booklet will contain all the answers you need.

Revision A: We hope the booklet will contain all the answers you need.

Revision B: Gazing at me hopefully, Sandy said, "May I have a cookie?"

I.E., E.G. Always be sure that you're using the correct one of these two Latin abbreviations:

• i.e. is the abbreviation for id est, and it means "that is."

• e.g. is the abbreviation for exempli gratia, meaning "for example."

• Of course, you could always just stick to the good old English equivalents – that is and for example.

INSURE, ENSURE Both of these words mean "to make secure or certain." Use insure if you’re writing about insurance, guaranteeing the value of life or property; use ensure other times when you mean "to make secure from harm."

ITS, IT’S, ITS’ Be sure to use the correct word:

• It's is the contraction meaning "it is."

• Its is the personal pronoun meaning "belonging to it."

• Its’ is NOT A WORD!

PLAN ON Plan on plus an "ing" verb is a colloquialism for plan to plus a verb.

Original: I plan on graduating next year.

Revision: I plan to graduate next year.

REASON IS BECAUSE Reason is because is a colloquial expression that people use when they mean reason is that.

Original: The reason is because our sales force is very aggressive.

Revision: The reason is that our sales force is very aggressive.

SET UP, SETUP Use the correct form.

• Setup is one word when it’s used as a noun or adjective:

Setup instructions were in the box. The setup took two hours.

• Set up is two words when it’s used as a verb:

We will set up for the meeting after work hours on Friday.

SHALL, WILL At one time, shall was used for first-person constructions and will was used for second- and third- person (I shall go; he will go). This distinction has largely passed from vogue, and now will is usually considered correct with all persons (I will go; he will go). However, shall is still used with all persons to emphasize determination, as in the following: Employees shall submit a written reason for all work absences.  The use of shall in contracts signifies a legal requirement.

TRY AND Try and is colloquial for try to.

Original: I will try and be there early.

Revision: I will try to be there early.

WHILE Use while to mean "during the time that." To avoid confusion, do not substitute while for connectives like and, but, or although.

Original: John is sales manager, while Joan is in charge of research.

Revision: John is sales manager, and Joan is in charge of research.

Original: While Joe wants the day off, he has not yet asked for it.

Revision: Although Joe wants the day off, he has not yet asked for it.

WHO, WHOM Use who as a subject (where you could substitute "he" or "they"); use whom as an object (where you could substitute "him" or "them").

Examples: Who is the leader? (Compare: He is the leader.)

Whom should we blame? (We should blame him.)

For whom did you order the pizza? (You ordered the pizza for them.)