[Grammar] I've written a letter since lunchtime.

[kadioguy]

Simultaneously?

continuously.

 

[Tdol]

Don't use the present perfect progressive here.

 

[jutfrank]

a. I've written a letter since lunchtime.
b. I've written a letter for an hour.
c. I've written six letters since lunchtime.
d. I've written six letters for an hour.
---------
Could you tell me which ones are incorrect or unnatural?

Could you tell me the reason?

I'd say that your questions are not useful. What do you mean by "incorrect or unnatural"? What is it you really want to learn?

You're coming at this from the wrong angle. A language form can only be correct insofar as it serves a certain purpose. They could all be correct in theory, in certain situations, (though a situation for c. is by far the most likely).

Why don't you just explain clearly the situation you imagine first, then we'll tell you the best way to say what you mean.