there must be a good reason for that...
How about rapidly? Nothing wrong with "fast" either. "Fast" can be an adjective or adverb in English.
"quickly" is the natural answer.
Quickly is the correct adverb to use here, or rapidly. Fast is an adjective, and is poor English in this context.
"fast" is both adjective and adverb ("he drives very fast") and there's nothing wrong with its use here, even if "quickly" might be more common.
"He undresses himself very fast" is not correct. Why?
We don't usually use "undress" reflexively. The most common is "he gets undressed"
"He undresses very fast" was not a correct answer in my case even though i spelled it exactly the same :(
works now, just checked it