Adjectives describe nouns; adverbs modify verbs, adjectives, and other adverbs. Confusing them — especially with -ly adverbs and flat adverbs — is a persistent intermediate error. Some words (fast, hard, late, early) look identical as adjectives and adverbs, while others have irregular adverb forms (good → well).
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The Basic Distinction
An adjective modifies a noun or follows a linking verb (be, seem, feel, look, smell, taste, sound): "The music sounds beautiful." An adverb modifies a verb ("She sings beautifully"), another adjective ("incredibly fast"), or another adverb ("really quickly"). A common error: "She speaks English good." ✗ → "She speaks English well." ✓ "Good" is an adjective; "well" is its adverb form.
Flat Adverbs — Same Form as Adjectives
Some adverbs have the same form as the adjective with no -ly added. These are called flat adverbs: fast, hard, late, early, high, low, straight, long, wrong, right, near, far, wide, loud. "She works hard." (adverb) vs. "It's hard work." (adjective). Most also have an -ly form with a different meaning: "lately" ≠ "late"; "hardly" ≠ "hard"; "nearly" ≠ "near".
The Linking Verb Trap
A common error is using an adverb after a linking verb. Linking verbs (be, seem, appear, feel, look, sound, smell, taste, become, get) are followed by adjectives, not adverbs. "I feel bad about it." ✓ (not "badly") "She looks tired." ✓ (not "tiredly") BUT: "She looked at me angrily." ✓ — here "looked" is not a linking verb but an action verb, so an adverb is correct.