Hi,
Just by chance I found the following reference to a
chapter in Njál's Saga, where the Imperative has been
used in a similar manner to the passage in "Hrólfs saga
kraka" that we were discussing. Because Njál's saga in a
way belongs more to "standard curriculum" than perhaps
Hrólf's saga does (more people are likely to have access to
a variety of translations of Njál's saga than to Hrólf's
saga, I believe), it might be a good idea to discuss the
example from Njál's saga first.

In some editions of Njál's saga, there is a numbering
system that is more or less standard. The example that
I have in mind is in Section 83. If your edition does
not have this numbering system, then please look a little
before the middle of the saga for a scene where they get
lost in the mists at sea and then end up somewhere on the
Scottish coast and are attacked by scottish Vikings who
want to kill them and take their merchandise. Well, the
example then goes like this :

« Mæl þú alz vesall. »

which is what the merchants reply after Helgi had answered
the Scottish ultimatum a bit too quickly for their taste.
(look here http://www.snerpa.is/net/isl/njala.htm and go to
83. kafli)

Here the literal translation is

"Speak you! completely poor/wretched."

which is of course not a very good English translation.
Mæl = 2nd singular imperative of mæla = 'to speak'.
þú = 2nd singular personal pronoun = 'you'.
alz = alls = adverb 'completely' or 'altogether'.
vesall = adjective singular masculine Nominative = 'poor'
or 'wretched'.

Finnur Jónsson, who comments this phrase, translates it as
"sei verwünscht für deine Worte" (=Be accursed for thy words),
which every one surely will agree is quite different from
the literal translation given above. Why is this so?

The reason, it turns out, is that the sentence "mæl þú alls
vesall" is an example of a so called "Imperativi Perfecti",
i.e. a past or perfected imperative, and it is a usage that
I have not seen in other languages such as German, Danish
or English. It is an imperative that is applied to an act
that has just been completed, hence its name.

In the example Helgi has just said that "the merchants want
to fight". But the merchants do not agree with this and think
Helgi has been a bit too quick in answering on their behalf.
Then *after* Helgi has spoken, they retort by saying "mæl þú
alz vesall", where mæl! (=speak) is the 2nd p. sg. imperative.
Thus, it is as if they issue Helgi a command after the fact.
They don't say "Speak Helgi!" *before* he speaks, but rather
command him to speak *after* he has already spoken words
they disapprove of. When the imperative is used in such a
manner after the fact, then it cannot be translated to English
by using the English imperative, but must rather be written
with words that look quite different from the literal translation.
Here Finnur Jónsson's translation is but one among several
possible schemes of translation.

In order to get a better feeling for the true meaning of
the phrase "mæl þú alls vesall!", I have tried to locate
as many different translations of Njál's saga as I could
find, and then compare the various translations.

Here is a list of some that I found:
F. Paasche "Tal deg usæl!" (speak yourself wretched!)
N.M. Petersen "Gid du må få skam af de ord!" (May your words
bring you shame!)
Aslak Liestøl "Måtte du få all verdsens ulykke for kjeften din!"
(May all the world's misfortunes be upon you for your big mouth!)
Finnur Jónsson "Sei verwünscht für deine Worte!" (Be accursed
for your words!)
A. Heusler "Sprich als gänzlich elender!" (Speak as an utter wretch!)
"Bekomm's dir schlecht, daß du so gesprochen hast!" (May ill
come over you, for speaking like that!)
G.W. DaSent "Wretch that thou art to speak thus!
(see http://sunsite.berkeley.edu/OMACL/Njal/5part.html for DaSent's
translation)

Conclusion:
When the ON imperative is applied to a just completed
act, it can have the meaning of a curse, if the attached
adjective has a negative connotation.

On the other hand, if the attached adjective is of positive
connotation, then the past imperative may express a blessing,
as I hope to show in another post.

Best,
Xigung