These are common. Consult a dictionary for other terms if you are not sure which word to use.
adviser/advisor, advisory — The one who advises is an adviser or advisor but serves in an advisory capacity.
affect, effect — Affect (verb) To influence. Rain affects crop growth. Effect — (verb) To accomplish or bring about. Those who wish to effect change must first make changes in themselves. Effect — (noun) Result. The effect of the medication was lower blood pressure.
afterward — No “s” at the end.
all right — The correct spelling is two words. Alright is not all right.
among, between — Among refers to three or more things. He is among the top 10 students in the class. Between shows the relationship of two things. The only difference between the twin sisters is the length of their hair.
are, our — The verb are is the 2nd person singular present and 1st, 2nd, 3rd person plural present of be. You are my sister. They are cowboys. The adjective our is a 1st person plural possessive form. Tim and I visited our parents. We returned and found our cars had been destroyed by the tornado.
assure, ensure, insure — A person assures (promises, reassures) others about something. He assured me the room was reserved in my name. When a person ensures something, he is making certain something will happen or securing an outcome. I made a deposit on the room to ensure it would be available when I arrived. A person insures something against damage or loss. The hotel insures its building and contents against damage from fire or other catastrophes.
backward — No “s” at the end.
based on, on the basis of — Using based on at the beginning of a sentence can be problematic because the phrase is often a dangling modifier. For example, Based on the results, we decided to . . . means that “we” are “based on the results,” which is not correct. For simplicity, use based on as a verb. On the basis of my experience . . . . Our conclusions were based on . . . .
because of, due to — Because of means as a result of. Becauseof our experiences in the workshop, we learned to develop a household budget. Due to means attributable to. The participants’ increased knowledge is due to their workshop experiences.
bi — The prefix indicates “two” or intervals of two units. Biweekly could mean twice a week or every two weeks, and bimonthly could mean twice a month or every two months. To avoid confusion, avoid using bi- in this sense.
biannually, biennially — Note the difference in spelling and meaning: biannually (twice a year) and biennially (every two years).
can, could — Can means am, is, or are able; it expresses ability and power. Could indicates possibility. (See may, might.)
choose, chose — Choose means to select. Choose wisely. Chose is the past tense of choose. You chose unwisely.
compare with, compare to — Compare with means to examine similarities and differences. The corn-based diet was compared with the soybean-based diet. Compare to means to focus on similarities. The 4-H volunteer compared the excited campers to a herd of cats; it was impossible to keep the group together.
disc, disk — Generally, CDs and DVDs are discs; other storage media are disks (hard disk, disk drive).
disinterested, uninterested — These words are not interchangeable. Disinterested means impartial. The mediator served as a disinterested observer of the proceedings. Uninterested means not interested. She is uninterested in the fortunes of the football team.
every day, everyday — Every day (two words) refers to something that occurs daily. The convenience store is open every day. Everyday (one word) means common or routine. It’s an everyday occurrence.
farther, further — Farther is used to express physical distance. He hiked farther into the wilderness. Further applies to extent or degree. I will conduct further research into the problem.
fewer, less — Use fewer if you are counting items. There are fewer marbles in this jar than in the other one. Use less to indicate an amount. He has less than $100 in his account.
flier — Standard spelling in American English, applies to both the person who flies and the handbill announcing an event or product.
impact — Avoid using impact as a verb, except to mean “strike forcefully.” Raindrops impacted the soil surface, causing runoff and erosion. Do not use impact to mean “affect.”
its, it’s — Spell the contraction of “it is” with an apostrophe (it’s). Leave out the apostrophe for the possessive of “it” (its).
lay, lie — Lay is an action verb and means to put something down. Its past tense is laid. Lay the book on the table. I laid the baby in the crib. The verb lie refers to a state of reclining or resting and its past tense is lay. You should lie down if you feel ill. He lay on the beach all day.
lead, led — Lead as a verb means to guide or direct. You can lead a horse to water. But as a noun, it is also the mineral. The past tense of the verb lead is led. He led the horse to water but couldn’t make it drink.
loose, lose — Loose means not tight. These pants are too loose. Lose is the present tense of lost. Don’t lose your keys.
may, might — May expresses permission or possibility. Use might to express something contrary to fact. If I had studied, I might (not may) have passed the test. (See can, could.)
over, more than — Over is generally used to describe spatial relationships and passage of time. Over the past 10 years, participation has increased. Use more than for numerals, figures, and amounts. More than half of the students passed the test.
than, then — Use than to introduce a second item for comparison. Joel is taller than Bob. Use then to express an element of time, often sequential. Mary went to the grocery store and then went home.
that, which — Use that for phrases that are essential to the meaning of the sentence and without commas. The house that has red siding is ours. Use which for a phrase that is not essential to the meaning of the sentence, and separate it with commas. The corner house, which has red siding, is ours. If you can leave the phrase out of a sentence without changing the meaning, use which. Otherwise, use that.
they, their— Do not use when referring to a singular person. When trying to avoid using he as a generic pronoun, rewrite the sentence to avoid the problem. Wrong: A visitor should park their car in the north parking lot. Correct: Visitors should park their cars in the north parking lot. See also agreement of pronoun and antecdent entry in the Style Guide section.
toward — No “s” at the end.
under way, underway — Use under way (two words) when you mean something is in progress or in motion. County fair plans are under way. Underway refers to something happening while in motion. The tankers are responsible for underway fueling of fighter jets.
use, utilize — Use is appropriate in most cases. Utilize suggests a new, profitable, or practical use.
various, varying — Various means different or unlike. The three treatments had various results. Use varying to mean changing. The varying cloud cover throughout the day made the photo shoot difficult.
where — Use to describe a physical place. Incorrect: In months where rainfall was frequent, irrigation was not applied. Correct: In months when rainfall was frequent, irrigation was not applied. Depending on the sentence, better choices might be when, whereas, or which.
while, since — Use to indicate passage of time. Depending on the sentence, better word choices might be although, but, and, whereas, or because.
while, since, although, whereas — While and since indicate time. Since last year, my writing has improved. Although and whereas indicate conditional relationships. Although yield was higher in treatment one, the difference was not significant.
who, whom — Who is used in the subject position of a sentence. Who is it?Whom is used in the object position of a sentence. To whom were you speaking? However, whom often sounds stilted in daily use, and many usage guides consider who to be standard English for both subject and object. Who do you wish to speak to? For formal writing (such as journal articles), follow the traditional rules for usage, but understand that informal language is sometimes acceptable.