Grammatical correctness alone is not enough to ensure that a sentence is effective and artful—or that it serves an appropriate purpose in your writing. Understanding grammatical structures can, however, help you produce sentences that are appropriate and effective as well as grammatically correct. The English language includes eight different categories of words called the parts of speech—verbs, nouns, pronouns, adjectives, adverbs, prepositions, conjunctions, and interjections. Many English words can function as more than one part of speech. When you book an airplane flight, the word book 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.