affect/effect: Affect is a verb that means "to influence": Counsel's explanation did not affect the hearing officer's decision. The noun effect means "a result": The effect of her decision was financially devastating. As a verb, effect means "to cause to come into being": The new chief thought he could effect a needed change in morale.
among/between: Among is used for three or more entities: Many local runners are among those training for this weekend's race. Between is used when two entities are involved: There is an intense rivalry between the two schools.
fewer/less: Fewer refers to people or objects that can be counted; less refers to general amounts: Fewer technicians means less work will be done today.
its/it's: Its is the possessive form: The board forwarded its recommendations. It's is a contraction of it is: It's time for a party.
lead/led: As a noun, lead is a kind of metal. As a verb, lead means "to conduct." A guide will lead a tour of the ruins. Led is the past tense of the verb "to lead": He led the firm for twenty years.
loose/lose: Lose is a verb whose past tense is lost: Did you lose your elevator card, again? Loose is generally an adjective for "not tight": I have a few loose screws.
maybe/may be: Maybe is an adverb: Maybe the negotiators will succeed this time. May and be are verbs: The rumor may be true.
passed/past: Passed is the past tense of the verb to pass: Everyone passed the test. Past means "beyond a time or location": The band marched past the bleachers.
principal/principle: As a noun, principal means "chief person" or "main or original amount": The principal of Waldorf's Feline Academy prefers titled cats. We paid only the interest on the principal. The adjective principle means "a basic truth": The parties have reached an agreement in principle.
who/whom: Who is used as a subject or subject complement: Who is the new guy in administration? Call Jennifer, who I know has the answer [who is the subject of the verb phrase "has the answer"]. Whom is used as an object: Bill, whom we all know, has just retired [whom is the object of the phrase "we all know"].