Explain the Difference Between Two Confusable Words

Affect or effect, whom or who, it is always the same handful of words that trip you up mid-sentence. This gives the rule, one trick that actually sticks, and three example sentences each, so the next email does not need a second guess.

Explain the difference between [affect and effect] in plain English. Give the rule for telling them apart, one memorable trick for remembering which is which, and three example sentences for each word showing correct use. Keep the whole answer under 150 words.