Some English word pairs look or sound almost identical but have different meanings and different grammar. These are the ones that cause the most mistakes in writing.
Affect is usually a verb meaning "to influence". Effect is usually a noun meaning "a result".
Memory trick: Affect is an Action, Effect is the End result.
Lose (rhymes with "choose") is a verb: to fail to keep or win. Loose (rhymes with "goose") is an adjective meaning not tight.
Advice (noun, uncountable) is the help you give: "Let me give you some advice." Advise (verb) is the action: "I advise you to wait." Say "a piece of advice", never "an advice".
Accept means to receive or agree: "She accepted the job offer." Except means "but not": "Everyone came except Tom."
A principle is a rule or belief: "It's a matter of principle." Principal means main or is the head of a school: "the principal reason", "the school principal". Trick: the principal is your pal.
Quite is an adverb meaning "fairly" or "completely": "The film was quite good." Quiet is an adjective meaning silent: "Please be quiet." Only the order of the last two letters differs, so proofread carefully.
Learn each pair as a contrast, not as two separate words, and create one example sentence for each. Check your writing for the part of speech: if you need a verb, you almost certainly need the other spelling than the one you would use for a noun.