All English words belong to one or more of eight grammatical categories called parts of speech: verbs, nouns, pronouns, adjectives, adverbs, prepositions, conjunctions, and interjections. Many English words regularly function as more than one part of speech. Take the word book, for example: when you book a flight, it is a verb; when you take a good book to the beach, it is a noun; and when you have book knowledge, it is an adjective.