Learn English – Why is “I’m doing great” correct

Most adjectives that describes the perceived ‘quality’ of an action carried out by someone, or their state of being, correspond to identical adverbs. Examples include such adjectives as ‘fine’, ‘good’, ‘bad’, ‘great’, ‘fantastic’, ‘horrible’, ‘okay’, etc.

I am doing great / †I am doing greatly
He plays okay / †He plays okaily
It works fine / †It works finely He’s got it bad / †He’s got it badly

Some, but not all, of these adjectives also have adverbial counterparts, whether formed by regular -ly derivation or otherwise, that are not identical to their adjectival forms. These are mandatorily used when describing degree/manner, rather than judging quality:

Wages have greatly increased
This story is fantastically boring
That tap is leaking badly

(Some dialects would be okay with using an adjective in the last example, but as far as I know, “Wages have great increased” is ungrammatical in every dialect of English, so the order of the constituents also plays a role here)

In the case of ‘good/well’, there is the additional problem that ‘well’ is both the adverb that corresponds to the adjective ‘good’, but also an adverb meaning “in good health”. This is part of the reason why so many know-it-alls insist that sentences like “I’m well” or “I’m doing good” are incorrect: they fail to realise that ‘good’ is an adverb and ‘well’ an adjective as well as vice versa. Moreover, the quality vs. degree/manner dichotomy does not fully apply to this particular pair either, further compounding the issue:

I am doing good / I am doing well

– are both equally valid, both when describing one’s current state of health (quality) and when answering a question on how one is getting along with a task (degree/manner).