Is there a difference between these two versions?
- He walks so slow.
- He walks so slowly
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There's no difference.
In He walks so slow, "slow" is an adverb formed by conversion from the adjective "slow".
The plain form, "slow", is not acceptable to everyone, though it's common enough. The inflected form, however, is more acceptable:
He is walking slower than usual. / This train is going slower than usual.
'Slow' is an adjective, 'slowly' is an adverb.
Adjectives describe nouns, while adverbs describe the manner in which an action (denoted by a verb) is carried out. Walking is a verb, so strictly speaking "he walks slowly" is the correct version, but you will hear native speakers say either because 'slow' has come to be used as a different kind of adverb called a bare or flat adverb. Not all adjectives can be used as a bare adverb, so an English learner should be careful to only use one when they know it to be idiomatic. Some style guides for professional writers may even advise against them.