Parts of speech is the most basic part of a language; any study of grammar must begin there. Most people learn at least something of parts of speech in school. Understanding parts of speech is prerequisite to more advanced grammar concepts.
What I’ve found is that, rather than memorizing a definition, it’s better to know what questions a part of speech can answer. For example, a noun answers the question “what?” or “who?” In, “The girl ate a cookie” girl is the “who” and cookie is the “what.”
Articles are the easiest. There are only three of them: a, an, the. Really, they’re just a subset of adjectives. Articles answer “which one?” (which is one of the adjective questions). “The dog” specifies which dog you’re talking about.
Adjectives describe a noun or pronoun. They answer the questions “what kind?” “which one?” “how many?” “how much?” and “whose?” These are words like tall, skinny, silly, three, eighteen, and few.
Adverbs describe a verb, adjective, or other adverb. They answer the questions “why?” “where?” “when?” “how?” “how often?” and “how much?” These are words like quickly, very, tomorrow, now, here, frequently, and severely.
Words aren’t the only adjectives. Phrases can be either adjective or adverb phrases. Take, for example, “The girl in the white dress bought a rose for her mother.” “In the white dress” is an adjective phrase, answering the question, “which one?” “For her mother” is an adverb phrase, answering the question “why?”
I actually had meant to make this post fun and interesting, and, since I haven’t really accomplished that, I will at least make it brief.