"Man kan lære å lese godt." Why is it godt and not god?
Because "godt" is functioning as an adverb here.
Why not use "bra"? I thought that was the adverb of "god"...
But what does it agree with?
Adverbs don't need to agree with anything, as they only have one form, but it's modifying "å lese".
Which is more formal of 'en' and 'man'? Do they infer anything different?
'en' is more formal/old-fashioned. They have the same meaning, but 'man' can only be used as a subject in a sentence.