How could you hand in a blank paper

[keannu]

Does this mean a past or a present mood?

M: Lucy, please sit down here. Look at your math grade! What's wrong?
W:Ah, the math...
M : You did a great job in most subjects, but math is the only problem. How could you hand in a blank paper for the math test?
M: I am really sorry for that.

[Rover_KE]

I'm not sure what you mean by 'mood', but as Lucy handed in a blank paper some time in the past...

Rover

[keannu]

1.Doesn't this have another possibility to be interpreted as shock or disbelief for a current event like "How could you do this to me(in the present)"? So do I have to tell depending on context?
2. I learned that "could" in the past denotes one's usual capability, while "was/were able to" does one' actual achievement, but considering this, such a rule is anything but useful. What do you think?

ex)How could you hand in a blank paper for the math test?

[billmcd]

1.Doesn't this have another possibility to be interpreted as shock or disbelief for a current event like "How could you do this to me(in the present)"? Not in the context presented. Handing in the blank paper (in the past) preceded the grade to which the teacher is referring (in the present). But I will say also that the reference could have been more clear if the statement read, "How could you have handed in etc."So do I have to tell depending on context?
2. I learned that "could" in the past denotes one's usual capability, while "was/were able to" does one' actual achievement, but considering this, such a rule is anything but useful. What do you think?

ex)How could you hand in a blank paper for the math test?

b.

[keannu]

Thanks a lot! Whether "how could~" refers to the past or the present seems to depend on context.
If anyone has any opinion about the second question, it's always welcome.

2. I learned that "could" in the past denotes one's usual capability, while "was/were able to" does one' actual achievement, but considering this, such a rule is anything but useful. What do you think?

ex)How could you hand in a blank paper for the math test?

[BobK]

Thanks a lot! Whether "how could~" refers to the past or the present seems to depend on context.
If anyone has any opinion about the second question, it's always welcome.

2. I learned that "could" in the past denotes one's usual capability, while "was/were able to" does one' actual achievement, but considering this, such a rule is anything but useful. What do you think?

ex)How could you hand in a blank paper for the math test?

Generally speaking, I find that rules that based on faulty and/or incomplete observation are anything but useful.

Let's try to work out what you were taught.

"could" in the past denotes one's usual capability -> Example: 'he could drive' (means 'he had the ability to drive'...)
but not 'he could choose to drive rather than to walk'?

"was/were able to" denotes one's actual achievement -> Example: 'The bridge was not closed, so he was able to drive'. But not 'The bridge was not closed, so he could drive'?

The 'rule' as you remember it is wrong. It is possible that usage may reflect some distinction between 'could' and 'was/were able to', but I'm not aware of it. Perhaps some other teacher knows which guideline you are misremembering.

b

[keannu]

I'm sorry I don't understand well, and I'd like to know why the following didn't happen and what the difference is.
ex)How could you hand in a blank paper for the math test?
->How were you able to hand in a blank paper for the math test?

[BobK]

How could you....?' is a simple exaggeration. This sort of hyperbole is commonly used to express disapproval. It is not asking 'In what way or to what extent were you capable of...?' It is simply asking 'Given your educatioon and your knowledge of the situation, what made you do this extraordinary thing? I am seriously disappointed.' (Of course, anyone is capable of handing in a blank exam paper - in this context 'could' has nothing to do with physical ability.)

b