"Content" words carry the most meaning in a sentence so receive more stress. They include questions words, nouns, most verbs, adjectives, adverbs, and negative auxiliary verbs (don't, won't, hasn't, etc).
"Function" words give a sentence its grammatically correct form. They carry little meaning so are unstressed. They include articles, pronouns, prepositions, conjunctions, the verb be, and affirmative auxiliary verbs (do, can, did, etc).
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