Usage, please study on page 894 in New Century Handbook

accept, except
adapt, adopt
advice, advise
affect, effect
allusion, delusion, illusion
among, between
amount, number
bad, badly
cite, sight, site
complement, compliment
conscience, conscious
continual, continuous
different from, different than
disinterested, uninterested
enthused, enthusiastic
farther, further
fewer, less
good, well
hopefully
imply, infer
its, it's, its
lay, lie
loose, loosen, lose
principal, principle
set, sit
stationary, stationery
than, then
their, there, they're
unique
who, whom
who's, whose
would have, would of
your, you're