a vs. an
Use ‘a’ when the first letter of the word following has the sound of a consonant. Keep in mind that some vowels sound like consonants when they’re sounded out as individual letters.
a finger
a hotel
a U-turn (pronounced You-turn)
a HUD program
a NASA study

Use ‘an’ when the first letter of the word following has the sound of a vowel.
Remember that some consonants sound like vowels when they’re spoken as individual letters.
an FBI case (F is pronounced ef here)
an honor (H is silent here)
an unusual idea
an HMO plan (H is pronounced aitch here)
an NAACP convention (N is pronounced en here)

accept vs. except
accept: to agree; to receive
except: but, with the exception that

aides vs. aids
aides: people who help; assistants
aids: helps, assists

adverse vs. averse
adverse: unfortunate; strongly opposed (refers to things, not people)
Examples: an adverse reaction to the medication
adverse weather conditions

averse: having repugnance (refers to people)
Example: He is averse to a military draft.

Advice vs. advise
advice (noun) recommendation
advise (verb) the act of giving a recommendation

affect vs. effect
Use effect when you mean bring about or brought about, cause or caused.
Example: He effected a commotion in the crowd.
Meaning: He caused a commotion in the crowd.

Use effect when you mean result.
Example: What effect did that speech have?

Use the verb affect when you mean to influence rather than to cause.
Example: How do the budget cuts affect your staffing?

Affect is used as a noun to mean emotional expression.
Example: She showed little affect when told she had won the lottery.

Aisle vs. isle
Aisle: passageway
Isle: a small island

all ready vs. already
all ready: all are ready
Example: We are all ready to go.
Already: refers to time
Example: Is it summer already?

All together vs. altogether
all together: refers to a group; all of us or all of them together
Example: It is wonderful to be all together to celebrate your birthday.
Altogether: entirely
Example: It is not altogether his fault.

Allude, elude, illude
Allude: to refer indirectly
Example: He alluded to his past as a spy.

Elude: avoid capture
Example: The fugitive eluded the police for a month.

Illude: mislead
Example: He illuded her about his age.

Allusion vs. illusion
Allusion: an indirect mention of something
Illusion: false perception

Ambiguous vs. ambivalent
Ambiguous: to have more than one meaning
Example: The law was ambiguous.
Ambivalent: to have mixed feelings
Example: She is ambivalent about her wedding dress.

Among vs. between
Among: involves three or more
Example: Who among us has not lied?

Between: involves just two
Example: She couldn’t decide between Chinese and Thai food.

Amount vs. number
Amount: used for things not countable
Example: We couldn't handle that amount of ill will.

Number: used for things that can be counted
Example: The number of accidents increased by ten percent.

Any more vs. anymore
any more: something additional or further
Example: It didn’t rain any more this year than last year.

Anymore: any longer, nowadays
Example: Harry doesn’t travel anymore.

Ascent, assent, consent
ascent (noun) movement upward
assent (noun or verb) enthusiastic agreement; to agree
consent: agreement

assumption vs. presumption
assumption: an idea not based on evidence
presumption: an idea based on evidence

can vs. may
can: able to
may: permission to

complement vs. compliment
complement: completing part of an order
compliment: praise

empathy vs. sympathy
empathy: to understand another's feelings
sympathy: to feel compassion or sadness for another

fewer, less, under
fewer: refers to a number that can be counted
Example: Fewer days off
Less: refers to an uncountable amount
Example: Less rain, less fear.
Under: used for direction
Example: Under the mattress, not under $100.

Imply vs. infer
Imply: to indicate without being explicit
Infer: to conclude from evidence

Irregardless vs. regardless
Irregardless: no such word exists
Regardless: in spite of, without regard

It’s vs. its

it's: contraction for it is or it has
Example: It’s for a good cause.
its: possessive pronoun
Example: The cat hurt its paw.

lay vs. lie
lay: Lay is a transitive verb, which means that it must be used with a direct object. The past tense of lay is laid.
Please lay the books on the table.
I laid the books on the table.

Have you ever seen a chicken lay an egg?
The chicken just laid two eggs.

"Now I lay me down to sleep..."
He laid himself down to sleep.

Lie: Lie is an intransitive verb, which means it cannot have a direct object. The past tense of lie is lay.
Lie down next to me.
I lay down next to her.

I just want to lie in bed all day.
Yesterday, he lay in bed all day.

Don't lie on the floor!
I lay on the floor last week and you didn't say anything.

Lie (past participle lied) means to say something untrue.
Don't lie to me.
He lied about where he got the money.

Set vs. sit
Set: one sets a thing
Example: Please set the table.
Sit: one sits oneself
Example: Please sit down at the table.

Than vs. then
Than: used for comparison
Then: indicates time, answers when

Their, there, they’re
Their: possessive pronoun
There: location
they're: contraction for they are

waiver vs. waver
waiver: a relinquishment of some right
waver: to feel indecisive; vary

Who’s vs. whose
who's: contraction for who is
Example: Who’s at the door?
Whose: possessive case of who
Example: Whose coat is this?