Adjectives and adverbs describe things. For example, compare the phrase “the bear” to “the harmless bear” or the phrase “they run” to “they run slowly.”

In both of these cases, the adjective (harmless) or the adverb (slowly) changes how we understand the phrase. When you first read the word bear, you probably didn’t imagine a harmless bear. When you saw the word run you probably didn’t think of it as something done slowly.

Adjectives and adverbs modify other words: they direct our understanding of things towards more specificity.

Adjectives and adverbs act in similar but different roles. Adjectives describe nouns and pronouns (“a reactive nanoparticle;” “she is proficient”); adverbs describe all the other parts of speech. Most intuitively, adverbs modify verbs (“our solution glowed brightly”), but adverbs can also modify adjectives (“a scarily reactive nanoparticle;” “she is thoroughly proficient”).

A lot of the time this difference between adverbs and adjectives can be seen in the structure of the words:

• A clever new idea.
• A cleverly developed idea.

Clever is an adjective, and cleverly is an adverb. This adjective + ly construction is a short-cut to identifying adverbs.

While –ly is helpful, it’s not a universal rule. Not all words that end in –ly are adverbs: lovely, costly, friendly, etc. (these are adjectives, e.g. a costly mistake). Additionally, not all adverbs end in -ly: here, there, together, yesterday, aboard, very, almost, etc.

Some words can function both as an adjective and as an adverb:

• Fast is an adjective in “a fast car” (where it qualifies the noun car), but an adverb in “he drove fast” (where it modifies the verb drove).
• Likely is an adjective in “a likely outcome” (where it modifies the noun outcome), but an adverb in “we will likely go” (where it modifies the verb go).