bad vs. badly

Bad is always an adjective and should be used to describe a noun; badly is always an adverb and should be used to modify a verb. The adverb badly is often used inappropriately to describe a noun, especially following a linking verb.

Example sentence with editing. Original sentence: The sisters felt badly when they realized they had left their brother out of their planning. Revised sentence: The sisters felt bad when they realized they had left their brother out of their planning.

Felt is a linking verb, so the adjective bad should be used to describe the noun sisters.

Exercises:

Adjectives and adverbs 1

Adjectives and adverbs 2

Adjectives and adverbs 3