Every word can be categorized as a part of speech based on its function in a sentence. In English there are usually eight parts of speech: nouns, pronouns, verbs, adverbs, adjectives, prepositions, conjunctions, and yikes! Interjections.
Dividing words into the parts of speech is a great way to understand how sentences work. In a nutshell, modifiers gotta modify — adjectives modify nouns and pronouns (brown bear), and adverbs modify verbs (runs quickly). Verbs are the heart of the sentence. No sentence is complete without one. Run! (That’s a complete sentence.) Prepositions show a relationship (under, over), and conjunctions (but, and) connect words and phrases. And dang, interjections (wow!) just want to have fun.