Is it an adverb or an adjective? Explaining words for a 9-year-old

Understanding adverbs vs. adjectives

Adjectives describe nouns (a person, place, or thing). Adverbs describe verbs, adjectives, or other adverbs, often telling how something happens.

Let’s look at your list one by one.

• Slow – typically an adjective when it describes a noun (a slow snail). Examples: The slow turtle walked slowly. Here, slow describes the noun turtle, so it’s an adjective. When it describes how something is done, it becomes an adverb (e.g., walk slowly).
• Nicely – this is an adverb. It describes how someone does something (how they acted). Examples: She spoke nicely. The word describes the verb spoke.
• Hasty – an adjective. It describes a noun (a hasty decision). Examples: That was a hasty choice. It does not describe how an action is done here.
• Nice – an adjective. It describes a noun (a nice day, a nice friend). It does not describe a verb in its basic use.
• Costly – an adjective. It describes a noun (a costly mistake, costly furniture). It tells about the noun, not how the action is done.

Quick recap: If the word describes a noun, it’s an adjective. If it describes a verb or tells how something is done, it’s an adverb. In your list: slow (can be adj or adv), nicely (adv), hasty (adj), nice (adj), costly (adj).