Commonly Confused Words

Accept / except
Accept is a verb meaning “to receive or come to terms with”—“She accepted his story as truth.” 
Except is either a verb meaning “to exclude”—“Please except that item from the list”—or a preposition meaning “excluding”—“I like every kind of ice cream except strawberry.”

A lot / allot
A lot is an informal way of saying “a bunch” or “several.”  It is never combined as alot (not a word). 
Allot is a verb meaning “to divide up and distribute”—“The father allotted one share to each child.”

Effect / affect
Effect is almost always a noun meaning “a result” or “a consequence”—“The drug had side effects.” 
Affect is a verb meaning “to change, alter, or have impact on”—“His harsh words really affected me.” 
Exception: effect can be used as a verb meaning “to bring about”—“The president’s new policies effected a big change in the economy.” (most often used in “to effect a change”)

Every day / everyday
Every day, typed as two words, means the same as each day or every single day. 
Everyday is an adjective that means “common,” “typical,” or “routine”: “The doctor said I could resume my everyday activities” or “Falling asleep at her desk is an everyday occurrence for her.”

Good / well
Good is an adjective used to describe a noun—“a good dinner” or “a good performance,” etc. 
Well is an adverb used to describe a verb—“she cooks well” or “the choir sings well.”  Well can also modify an adjective—“a well-known cook” or “well-informed journalist.” (Note the hyphen usage.)

It’s / its: 
It’s includes an apostrophe ( ‘), which means the word is a contraction of two words: it and is.  If you use this word, you must be able to logically replace it with “it is.”  Otherwise, it’s incorrect. 
Its (without an apostrophe) is a possessive pronoun which shows that something belongs to “it”—“The dog found its bone” or “The corporation failed to fully report its losses.”

Loose / lose
Loose (with the double o) is an adjective meaning a variety of things: relaxed, not tight, not secure, etc: “She preferred to wear loose-fitting pants” or “The loose screw made the ladder dangerous.” 
Lose is a verb meaning to misplace, shed, or not win; you can lose keys, lose weight, lose a game, etc.

Then / than
Then is an adverb referring to time or sequence (like “next” or “afterward”). 
Than is a conjunction implying a comparison—“He is a stronger swimmer than she is.”

They’re / their / there: 
They’re also includes an apostrophe, indicating that the word is a contraction of they and are.  Any other use is incorrect. 
Their is a possessive pronoun.  Use this word to show that something belongs to two or more people—“Their son failed first grade” or “The women fought for their rights.” 
There is an adverb denoting a place—“Put the book over there”—or an expletive, an “empty” word used to begin a sentence when the subject follows the verb: “There is too much poverty in our world.”

Too / to / two
Too is an intensifier used to show the extent or extremity of some adjective or adverb--“She’s too skinny to be healthy” or “He exercised too soon after eating dinner.” 
To is a preposition for location or direction—“Go to New York” or “Give it to the teacher.”
Two is a number following one and preceding three.

Weather / whether
Weather refers to the climate, temperature, outdoor conditions, etc.  
Whether is a conjunction referring to the choice between two or more alternatives—“I have to decide whether or not I want to attend the party.”

Who / whom
Who is a relative pronoun that acts as a subject, which means it usually appears just before a verb—“The prize went to the contestant who ran the fastest” or “Do you know who called this morning?” 
Whom is a relative pronoun that functions as an object of a verb or preposition—“This is the woman whom he photographed” or “I don’t know whom to contact” or “To whom would you like to speak?” (he photographed whom)  (to contact whom)   (to speak to whom), etc.  In each example, whom functions as an object (“receiver”) of something, not the subject (“doer”).

You’re / your
You’re is always a contraction of you and are (as noted by the apostrophe). 
Your is a possessive pronoun showing that something belongs to a second person: “You haven’t finished your dinner” or “Record your name in the space provided.”