well

well and good

Well and good can have similar meanings, but in this case well is an adverb, while good is an adjective. Compare:

• The car runs well. (adverb modifying runs) (not The car runs good.)
  It’s a well-made car. (adverb modifying made) It’s a good car. (adjective modifying car)
• He teaches very well.
  I like that teacher. He’s good. (not He’s well.)
• She speaks English well. (not She speaks English good.)
  She speaks good English. Her English is good.

Note that we cannot say She speaks well English. (Adverbs cannot usually go between the verb and the object, (see here).)

well = ‘in good health’

There is also an adjective well, meaning ‘in good health’.

• ‘How are you?’ ‘Quite well, thanks.’
• I don’t feel very well.

Note that the adjective well is only used to talk about health. Compare:

• When I’m in the mountains I am always well.
• When I’m with you I’m happy. (not When I’m with you I’m well.)

Well is not common before a noun. We can say She’s well, but it is less usual to say, for example, She’s a well girl.

I’m good.

I’m good is now often used informally to mean ‘I’m OK’, especially when refusing an offer.

• ‘More coffee?’ ‘No, I’m good, thanks.’
• ‘Want help?’ ‘It’s OK – I’m good.’

note

For ill and sick, (see here).

For well as a discourse marker, (see here).