'Good' is an adjective and describes nouns. 'Well' is an adverb and describes verbs. The main distinction is whether you are describing a thing or a person ('good') or describing how an action is done ('well'). Many mistakes come from using 'good' to describe actions or results, where English prefers 'well'.

Rule
Use 'good' to describe a noun.
Use 'well' to describe how a verb is performed.
Use 'well' to describe the result of an action.

'Good' qualifies a noun by saying it has positive qualities. It answers questions like 'What kind?' or 'Which one?' about a person, place, thing, or idea. In English, 'good' often appears before a noun or after linking verbs like 'be' when it refers to qualities, not actions.

Word/Phrase	Definition
good	'Good' is an adjective that describes a noun as having positive qualities.
a good book	A book that has positive qualities.
She is good	She has positive qualities or skill, not how she is acting right now.

'Well' modifies a verb by explaining how something is done. It answers 'How?' about an action. In standard English, 'well' is preferred when you are talking about doing something with skill, effectiveness, or success.

Word/Phrase	Definition
well	'Well' is an adverb that describes how an action is performed.
sings well	Performs singing in a skillful or effective way.
did well	Performed an action with success or effectiveness.

With linking verbs like 'be', English usually treats the complement as an adjective, so 'good' is standard when describing qualities. With action verbs, English expects an adverb, so 'well' is standard when describing performance. The confusion often happens with verbs that can act as linking verbs or action verbs in different contexts.

Rule
With 'be', use 'good' to describe qualities.
With action verbs, use 'well' to describe performance.
With 'seem' or 'look' as linking verbs, use 'good' for qualities.

'Well' can also mean 'in good health' and functions like an adjective in this context. English speakers say 'I am well' to mean healthy, and 'I am good' to mean fine or OK in a social sense. This is a fixed distinction in standard English, even though informal speech may blur it.

Word/Phrase	Definition
I am well	I am in good health.
I feel well	I am experiencing good health.
I am good	I am OK or fine in this situation.

'Good' describes nouns and qualities, especially with linking verbs. 'Well' describes how actions are performed and often appears with verbs to explain skill or success. For health, 'well' means healthy, while 'good' is a social response. Keeping the adjective versus adverb distinction prevents most errors with these words.