How can you sing so well?

keannu said:

After seeing a girl so talented in singing, which you have never expected,

1.How could you sing so well? => It's surely disbelif of surprise
2.How can you sing so well? => Is this also a surprise or "trying to investigate how the fact can happen"?

Does "can" focus more on the causes like "when you did start singing?" or "Did you get some lesson from professional teachers", etc?

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keannu said:

I might have taken an inproper example. My intention was to compare "can" and "could" in such similar cases where "could" means doubt or surprise. If you can tell the difference that I intend, please let me know.

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emsr2d2 said:

"How can you sing so well?" sounds to me like a genuine enquiry as to the physical requirements of singing well. Did you have training? Is it a natural ability? Or possibly surprise if followed by "... after drinking 5 pints of beer and smoking 40 cigarettes last night?!"

"How could you sing so well?" (rather unnatural) almost sounds like a complaint, as if the speaker is asking how that person dared to sing so well, like "You sang really well! How could you?! I hate you! I wanted to win the singing competition but now I have no chance."

I agree with bhaisahab that the particular sentences are not very natural.

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keannu said:

ex1)Ben! How could you? After all they've done for us! Disbelief that Ben finds it acceptable to do whatever it was. It sounds like whatever Ben did was a one-off event.

ex2)Ugh! How can you eat that stuff? A genuine question meaning something like "How are you able to eat that stuff without feeling sick?" It sounds like a habit, rather than a question to someone who is eating something for the first and possibly the last time.

ex3)How could you bring yourself to do a thing like that? (No answer is expected) This is rather unnatural thing to say if no answer is expected. The use of "bring yourself to do" gives a different connotation to the sentence.

Looking first at the broken lamp and then at the cat, Mary shouted, "How could you do that?" I would say this is a cross between disbelief and anger. Of course, the cat didn't break the lamp on purpose so it can't be held responsible but it's the automatic reaction of many people.

Tom: Then I punched him in the nose. Rachel: Oh, how could you? This could be disbelief, surprise or disappointment depending on Rachel's tone of voice.

I have separated out those three because initially I thought it was all one occurrence and conversation, but that didn't make sense.

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keannu said:

That's what I wanted and I need more generalization than specific examples. Now I can conclude that if you use "can", it is questioning "how are you able to..", really curious about the way something is done, while "could" is "disbelief" or "suprising" to mean "I can't believe or it's surprising that you did this".

This is my assumed example.
A: I got [STRIKE]a full score[/STRIKE] full marks on my math test today. Actually so far my average score for the last few semesters was only 70.
B : How can you do that? (I'm really curious about how you achieved it. Have you learned from a very competent tutor
or did you go overnight everynight to master math? What's the secret?)
Unnatural question. The first part "I got full marks" indicates the past, so "How can you" is not appropriate. You would ask "How did you do that?"

How could you do that?(I can't believe you did it. You might have cheated on the test or got a lucky score
with your guessed answers) No. This is the same kind of "How could you?" that I mentioned in my first response about the singing competition. It suggests that person B is really annoyed that person A got full marks in the test. Again, the correct question here would simply be "How did you do that?"

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