Words that sound alike or nearly alike but have different meanings and spellings are called homophones. The following sets of words are commonly confused. A careful writer will double-check their every use. (See also the glossary of usage, W1.)
affect (verb: to exert an influence)
effect (verb: to accomplish; noun: result)
its (possessive pronoun: of or belonging to it)
it’s (contraction of it is or it has)
loose (adjective: free, not securely attached)
lose (verb: to fail to keep, to be deprived of )
principal (adjective: most important; noun: head of a school)
principle (noun: a fundamental guideline or truth)
their (possessive pronoun: belonging to them)
they’re (contraction of they are)
there (adverb: that place or position)
who’s (contraction of who is or who has)
whose (possessive form of who)
your (possessive pronoun: belonging to you)
you’re (contraction of you are)