I have been corrected for using "Please, can you tell me exactly what happened?" instead of saying "Please, tell me exactly what happened?" I was told that using can you changes the meaning of the question. I cannot find anything that supports their claim. Can you provide me with the proper usage and why?
A person who is being needlessly, boorishly pedantic could take the absolute most literal meaning of "can you" as "are you capable of" and reply simply, "Yes." This person is essentially saying, "I did witness the events and I am able to communicate using human speech."
However, social convention provides a more loose interpretation of "can you" as "are you capable of, and if so, please do."
A similar situation arises in the common scenario of two strangers crossing paths on the street and one asking the other, "Excuse me, can you tell me the time?" This allows the response of "I'm sorry, no," without necessarily having to add, "because I do not have a watch or do not know how to tell time." Of course, this convention also allows for the answer, "Sure, it is 3:15," without having to preface it with anything.