Affect vs Effect — Affect = verb (to influence). Effect = noun (the result). "The cold weather affected her mood. The effect was that she stayed indoors all day." Memory: Affect = Action (verb). Effect = End result (noun).

Their vs There vs They're — Their = possessive pronoun (belonging to them). There = place or existence. They're = they are. "They're going to park their car over there."

Your vs You're — Your = belonging to you. You're = you are. "You're going to love your new house." Test: substitute "you are" — if it works, use you're.

Its vs It's — Its = belonging to it (possessive). It's = it is / it has. "The dog wagged its tail. It's a very friendly dog." Test: substitute "it is" — if it works, use it's.

Then vs Than — Then = at that time / next in sequence. Than = used in comparisons. "First we ate, then we left. She is taller than her brother."

Lie vs Lay — Lie (intransitive) = to recline; to be in a horizontal position (no object needed). Lay (transitive) = to place something down (needs an object). "I need to lie down. Please lay the book on the table."

Principal vs Principle — Principal = (noun) head of a school; (adjective) main or most important. Principle = noun, a fundamental rule or belief. "The principal reason is our company's core principle of honesty."

Complement vs Compliment — Complement = something that completes or enhances. Compliment = a flattering remark. "The wine complemented the meal perfectly. He complimented her on her dress." Memory: complEment = Enhance. complIment = I like you.

Accept vs Except — Accept = to receive or agree to something. Except = excluding; but. "I accept all your terms except the last clause."

Advice vs Advise — Advice = noun (the recommendation). Advise = verb (to give advice). "My advice is to rest. I advise you to see a doctor." Memory: Advice = noun (like ice — a thing). Advise = verb (ends in -ise like other verbs: supervise, revise).

Lose vs Loose — Lose = verb (to fail to keep something; to be defeated). Loose = adjective (not tight; free). "Don't lose the key. The screw is loose."

Stationary vs Stationery — Stationary = not moving; still. Stationery = paper, pens, office supplies. Memory: stationary = are you standing still? stationery = envelope (both contain e).

Fewer vs Less — Fewer = for things you can count (countable nouns). Less = for things you cannot count (uncountable nouns). "Fewer cars on the road means less traffic."

Imply vs Infer — Imply = to suggest something indirectly (the speaker implies). Infer = to draw a conclusion from evidence (the listener infers). "She implied she was unhappy. I inferred she wanted to quit."

Disinterested vs Uninterested — Disinterested = impartial; having no personal stake. Uninterested = bored; having no interest. "We need a disinterested judge. He was completely uninterested in the topic."

Emigrate vs Immigrate — Emigrate = to leave your country to live elsewhere. Immigrate = to arrive in a new country. Memory: Emigrate = Exit. Immigrate = come In. "She emigrated from Poland and immigrated to Canada."

Elicit vs Illicit — Elicit = verb, to draw out a response or reaction. Illicit = adjective, illegal or forbidden. "The speech elicited strong emotions. He was involved in illicit activities."

Further vs Farther — Farther = physical distance (measurable). Further = figurative/additional (non-measurable). "London is farther than I thought. We need to discuss this further." Memory: farther has far in it — physical distance.

Historic vs Historical — Historic = famous or significant in history. Historical = relating to history or the past. "It was a historic speech. I enjoy historical novels."

Adverse vs Averse — Adverse = unfavorable, harmful (describes conditions). Averse = having a strong dislike or opposition (describes a person). "Adverse weather delayed the flight. She is averse to taking risks."

Continuous vs Continual — Continuous = without any interruption. Continual = repeated frequently but with breaks. "The machine makes a continuous hum. The continual interruptions frustrated her."

Ensure vs Insure vs Assure — Ensure = to make certain something happens. Insure = to take out insurance. Assure = to tell someone confidently to remove doubt. "I assured her it was safe. Please ensure the door is locked. Insure the car before driving."

Flaunt vs Flout — Flaunt = to display proudly and ostentatiously. Flout = to openly disregard a rule or convention. "She flaunted her new watch. He consistently flouted the company rules."

Mitigate vs Militate — Mitigate = to make something less severe. Militate = to have a strong effect against something. "The medication helped mitigate the pain. The evidence militates against his innocence."

Pore vs Pour — Pore (verb) = to study or read something carefully. Pour = to flow liquid. "She pored over the documents all night. He poured himself a glass of water."

Who vs Whom — Who = subject (the one doing the action). Whom = object (the one receiving the action). Test: substitute he/she (who) or him/her (whom). "Who called? Whom did you call?"

Envy vs Jealousy — Envy = wanting something that belongs to someone else. Jealousy = fear of losing something (or someone) you already have. "I feel envious of her success. He felt jealous when she talked to other people."

Bi-weekly — what does it mean? — Ambiguous! It can mean either twice a week OR every two weeks. Avoid it in formal writing — use "twice a week" or "every two weeks" to be clear. Same issue with bi-monthly, bi-annually.

Nauseated vs Nauseous — Traditionally: Nauseous = causing nausea. Nauseated = feeling nausea. "The smell was nauseous. I felt nauseated after the ride." (Modern usage accepts "I feel nauseous" to mean feeling sick.)

Masterful vs Masterly — Masterly = performed with great skill. Masterful = powerful, domineering; commanding. "It was a masterly performance. He has a masterful personality."