Frequently-confused words
Here are a few more cases of often-confused or misused words.
censor / censure: Censor means to edit or remove writing or art that someone dislikes; it can also mean a person who does this. Censure means an official reprimand, or the act of imposing one.
click / clique: A click is a type of sound; a clique is a tight-knit and exclusive group of people. I have seen "click" used to mean "clique", but this is wrong.
diffuse / defuse: Diffuse means vague or thinly-spread, and can (rarely) be a verb meaning to spread out thinly, as in "the gas diffused through the atmosphere". Defuse is what you do to a bomb to stop it from exploding, and can also be used figuratively to mean calming down a tense situation, as in "the mediator defused the conflict before it escalated into war".
discreet / discrete: Discreet is roughly "secretive" or "confidential", as in "you must handle this discreetly -- don't let anyone know what we're doing". Discrete is a rare word meaning separate or distinct, as in "this process has several discrete steps".
disperse / disburse: Disperse means to thin out or break up something, as in "the police dispersed the crowd", or sometimes to spread out, as in "the immigrants dispersed through the country instead of all settling in one place". Disburse means to give out money. The two words are often confused but are quite distinct.
elicit / illicit: To elicit means to evoke, as in "the speech elicited strong feelings from the audience". Illicit is an adjective meaning illegal or unethical, as in "a scandal erupted over the politician's illicit behavior".
etc: This is short for the Latin et cetera, meaning "and so on in the same way". Spelling it "ect" is incorrect.
faze / phase: Faze is a verb and means to daunt or discourage, and is most often used in the negative -- "he was unfazed by their criticism". Phase is a noun meaning a stage or period within a larger process; it is also sometimes used as a verb indicating some kind of transition, as in "the program was phased out". I often see "unphased" used in place of "unfazed", but this doesn't mean anything, unless it means somebody has not been shot with a Star Trek phaser.
kudos: This word is singular, not plural. There is no such thing as "a kudo".
moral / morale: Moral means roughly "ethical" and can also refer to ethical standards, as in "he has good morals". Morale (stressed on the second syllable) means roughly "mood" or "spirit", usually of a group of people; "the soldiers' morale was high as they marched into battle".
pedal / peddle: Pedal is what you do on a bicycle; to peddle is to try to sell something. These are sometimes confused in the compound word "backpedal", meaning to retreat or back away, incorrectly written as "backpeddle", which doesn't exist but would presumably mean trying to sell something back to the person you bought it from.
replete / complete: Replete with means "full of", with a strong implication of being overstuffed. It isn't just a fancier version of "complete with" -- it doesn't mean the same thing and should not be used in place of "complete with".
tenant / tenet: A tenant is a person who lives in rented housing; a tenet is an individual idea which is part of a belief system, for example, "the supremacy of the pope is one of the tenets of Catholicism".
Finally, I should mention the use of early modern English grammatical endings like -th and -st, and pronouns such as thee and thou, to make language sound old-fashioned. These have standard usage and cannot just be thrown around willy-nilly. The -th ending is just the older form of the -s which modern English attaches to verbs with a third person singular subject; he worketh, she hath seen it, his behavior annoyeth me, etc. Saying, for example, "I hath" is ungrammatical; it's exactly the same as "I has". The -st ending goes with the subject pronoun thou; thou workest, thou hast seen it, etc. Unlike other personal endings, it is also applied to the past tense; thou didst do it, thou gavest it to me, etc. This ending is only used with thou.
Thou is the old familiar/informal form of "you". Thou is used for a grammatical subject, thee for an object, thy or thine for possessive, with the distinction being like "my" vs "mine". Thus, thou art welcome here, I can see thee, I hear thy words, everything here is thine. All of these are singular only and can never be used to refer to more than one individual, in contrast to the modern "you" which can be used for one person or several.
If you found this post helpful, please check out the earlier one as well, for more such cases.