OP unwittingly chose two excellent examples of where he thinks the two words are "interchangeable"...
1: I have nothing more to say on the subject.
2: The party was divided on the issue.
These totals from Google Books are relevant...
more to say on the subject 98,400 hits
more to say on the issue 5,220
divided on the subject 475,000
divided on the issue 604,000
the subject was resolved 7,430
the issue was resolved 106,000
Here's a link showing how divided on the issue has become far more common in recent decades, and here's another one showing that a popular subject remains the more common choice.
Putting it all together, where the sense for both words is area of discussion...
1: issue meaning topic, subject has only really taken off in the past half-century.
2: issue is very strongly associated with contentious topics (more argued over than discussed).
See StoneyB's comment below for more on how "legalese" usages such as the question at issue arose, eventually leading to what now looks like "near synonymy" in current popular use. Also note this trend.
In general, questions in English must include one of the following:
- a helping (auxiliary) verb, such as have, will, and would,
- a form of the verb be,
- or a form of the verb do as an auxiliary verb, if neither of the above is present. In this case, the main verb is not modified by the subject or tense. "Does she drive?" is correct, while "Does she drives?" is not. "Did she drive?" is correct, not "Did she drove?"
In any past tense, the helping verbs are simply conjugated in the tense desired, such as did, were, was, had, etc.
Let's apply these rules to your examples.
How many pegs did you have?
This is correct, because it includes the helping verb do in past tense.
How many pegs did you drink?
Also correct, for the same reason as above.
How many pegs you drank?
This is incorrect; it does not have either a helping verb, a form of be, or a form of do. It is actually corrected by the sentence below it.
How many pegs have you drank?
This is correct, since the helping verb have is present, and drink is correctly put into the past participle form drank. This is how the sentence above it should read.
How many pegs you have had?
This is close, but the subject and verb should be inverted to form "How many pegs have you had?"
Best Answer
This sounds like it should be part of a sentence explicitly explaining the differences between the two. (although it sounds a bit clumsy admitedly)
This could be used much the same, however, it could also just be a statement remarking that there is a large difference (especially adding the exclamation mark).