Though they are frequently used words, good and well are often confused. The basic rule of thumb is that good modifies nouns and pronouns, while well modifies verbs and adjectives. However, when you factor in linking verbs and the adjective version of well, things get a little stickier.
good, adj. virtuous, proper, of high quality, well-behaved, kind, honorable, etc.
n. benefit, kindness, virtue
well, adv. in a satisfactory manner, thoroughly, adequately
adj. in good health
Let’s start with the easy ones.
Joe ate good food.
-the adjective good modifies the noun food, which tells us the food was not bad.
Joe ate well
-the adverb well modifies the verb ate, which tells us the way Joe ate (without a tantrum, probably).
What if we use a linking verb instead of an action verb?
Should it be “Joe is good” or “Joe is well“?
That depends on the meaning of your sentence. Since linking verbs act as an equal sign, they’re often followed by predicate adjectives which tell the reader more about the subject (also called a subject complement).
So if you intend to say that Joe is a kind or honorable person, you would choose, “Joe is good.” Whereas if you intend to say Joe is in good health, you would choose, “Joe is well.”
It gets tricky when we use verbs which could be linking verbs or action verbs.
Both “Joe looked well” and “Joe looked good” can work, depending on what you’re trying to say.
“Joe looked well” can mean he appeared to be in good physical health or he did a thorough job of looking.
“Joe looked good” means he was attractive or well-dressed.
Another great example of the difference between good and well shows up with the verb “do.”
“Joe does well” and “Joe does good” have different meanings. The first tells us either how Joe performs as task or (idiomatically) that he is wealthy. The second tells us that Joe is involved in some sort of charity work: volunteering, donations, organizing, etc.
Why does it matter?
Clarity of communication. While native speakers of English will probably understand your meaning with either word, you shouldn’t underestimate the power of picking the right word.