Good and well both express positive evaluation, which is why they are often confused. The main difference is grammatical: good is usually an adjective, while well is usually an adverb. This distinction is closely tied to Adjectives and Adverbs, and it affects how each word fits in a sentence.

Good describes nouns and can follow linking verbs when the sentence describes a state, quality, or judgment. It is also the normal choice before a noun, as in a good idea, and it appears in common patterns such as good job. In informal American English, I'm good often means I am fine, which is a fixed everyday meaning rather than a literal description of quality.

Idea	Example
Good describes a noun.	She has a good idea.
Good follows linking verbs to describe a state.	The meal is good.
Good comes before a noun.	We need a good plan.

Well describes how an action is performed, so it normally modifies verbs. It also has an important adjective meaning that refers to health, especially after verbs like feel, look, and appear when the meaning is physical condition rather than performance. In praise expressions, well done is common and polite, and it often sounds a bit more formal than good job.

Idea	Example
Well describes an action.	He sings well.
Well can mean healthy.	I feel well.
Well after look can mean health.	She looks well.

Use well when the word modifies a verb, and use good when the word describes a noun or a state. After perception verbs such as look, feel, and appear, the choice depends on meaning: good usually points to appearance or general approval, while well points to health. This same pattern helps explain why prescriptive grammar prefers did well, even though did good is common in casual speech.

Idea	Example
Use well with verbs.	He works well.
Use good with nouns.	That was a good answer.
Use well for health.	He does not feel well.
Use good for appearance or approval.	She looks good today.

Some fixed expressions use both words to praise performance, but they differ slightly in tone and formality. Good job is direct and common in everyday speech, while well done often sounds more polished or ceremonial. Comparative forms also follow the same pattern, with better and best serving as the usual comparative and superlative forms for both words.

Idea	Example
Good job is a praise expression.	Good job, you finished early.
Well done is a praise expression.	Well done, you solved it quickly.
Better is the common comparative.	Her result is better.
Best is the common superlative.	This is the best choice.

In informal American English, I'm good often means I am fine or I do not want anything more. In careful standard English, did well is preferred for performance, while did good may appear in casual conversation or when good means charitable or morally helpful. Look good usually refers to appearance or approval, while look well usually refers to health.

Region	Word or Phrase	Regional Definition	Example
American English	I'm good	This often means I am fine or no thank you in casual speech.	I'm good, thanks, because I already ate.
Informal Speech	Did good	This can appear in casual speech, though did well is the standard performance form.	He did good in the game, because everyone was cheering.
Standard English	Did well	This is the preferred form when describing performance.	She did well on the test, because she studied.
Health Context	Look well	This means to appear healthy rather than attractive.	He looks well, because he has recovered.

The simplest decision rule is this: use good when describing a noun or a state, and use well when modifying a verb. When a linking verb or perception verb creates a choice, decide whether the meaning is quality, appearance, or health. Once that distinction is clear, the difference between good and well becomes predictable in everyday English, especially alongside Comparative Adjectives and Comparative Adverbs.