Can an adverb modify another adverb in Chinese?
we don't say 你太快说

你说得太快

means you speak too fast

你快说

expresses that I want you to speak faster, it's like a request.
你快说 can be used on informal occasions (with your friends or your family). We don't say it to people you are not familiar with, they would think you are being impolite and inpatient.
You can say &quot;你可不可以快点说？“ This sentence is better than 你快说. It's more polite and good attitude.