Like many niggling details in grammar, the difference between good and well is both simple and complex. Basically, good is an adjective, while well is an adverb, but the usage of these words gets much more complicated than this, thanks to a little detail known as linking verbs. If you've been tormented for saying “I'm good,” you may be pleased to know that this usage is actually correct, and that the difference between good and well is often oversimplified by pedantic individuals.

Let's start with the simple part. Good, like other adjectives, is used to modify a noun, providing more information about it. For example, one could say “Bronwyn's dog is good” or “it's a good house, very solidly built.” Well, acting as an adverb, is used to modify verbs, like this: “he sings well,” or “the cat hunts well.” When thinking about whether to use “good” or “well” in a sentence, think about whether you are describing an action, like playing the piano or hitting a baseball, or an object, like a garden or an anvil.

However, there is an exception to this simple rule of thumb for using good and well. Good can be used in sentences like “I'm good” or “the pizza looks good,” because the verbs involved in these sentences are linking verbs, connecting a subject with information about it. A number of words can be used as linking verbs, including seems, appears, looks, grows, tastes, becomes, and the various conjugations of “to be,” including am, is, are, was, and so forth. Some of these words can also be used as action verbs, but when used as linking verbs, they do not imply action, they simply connect subjects and information.

If you're starting to feel confused about how to use good and well when linking verbs are involved, there is fortunately a very easy way to tell when a verb is being used as a linking verb, and that is to replace it with am, is, or are, as appropriate. If the sentence turns into gobbledygook, the verb is being used as an action verb, in which case “well” is appropriate, but if the sentence still makes sense, you should use “good.”

For example, “the pasta looks good” sounds perfectly normal when you replace “looks” with “is” to make “the pasta is good,” so good is an entirely appropriate word to use. However, in the sentence “the bunny smelled the grass but decided not to eat any,” when you replace “smelled” with is, the sentence doesn't make sense, so “smelled” is not being used as a linking verb in this case, so you would say that the bunny smelled the grass well, not good.

Many people tend to over-correct when struggling with the usage of good and well, but you shouldn't need to if you can remember that good goes with nouns and well goes with verbs, unless the verbs are being used in a linking capacity.