There are a few adverbs and adjectives that cause a lot of trouble. Let’s focus on the chief troublemakers:
To see which one you’re dealing with, you have to look at the rest of the sentence. Take the following examples:
He treated me badly.
That was a bad idea.
He treated me well.
That was a good idea.
So far so good: the adverb modifies the verb and the adjective describes the noun. The difficulty comes when we use a linking verb:
She felt bad about our breakup.
A linking verb is usually followed by a noun or adjective that describes the subject. That’s why we have to use bad rather than badly. You can see why this is so if you compare a similar sentence:
The weather forecast looked bleak.
The linking verb itself is not being modified—it’s the forecast that’s bleak.
But there is one more thing to watch out for: the adverb well can also be an adjective, in which case it refers to one’s health. Compare these sentences:
After her fever subsided, Belinda was well enough to go to class.
After her fever subsided, Belinda felt good enough to go to class.
In each case we’ve used an adjective to describe Belinda’s health.