The paradigm of the verb 'kalla' that was posted here last night
contains a number of odd features which are not part of the modern
Icelandic language, and aren't familiar to me from medieval texts.
For example, the 3rd person indicative and subjunctive plural forms
seem to be the wrong way round, the 2nd/3rd person singular indicative
'kallar' has been printed in place of the imperative singular:
'kalla', 'kallaðu' (=kalla + þú). And u-umlaut is missing in the 2nd
and 3rd person plural past tense, where we would expect: 'kölluðuð',
'kölluðu', etc.

I would suggest that anyone trying to learn Icelandic / Old Norse verb
conjugations look elsewhere. You can find the early Old
Norse/Icelandic paradigm for 'kalla' here:

http://www.utexas.edu/cola/centers/lrc/eieol/norol-9-R.html#Nor09_GP41

And the Modern Icelandic paradigm here:

http://www.lexis.hi.is/beygingarlysing/so/vb/kalla.html

In fact you can use this site to check the inflections of other Modern
Icelandic words [ http://www.lexis.hi.is/beygingarlysing/ ]. 'leita'
= "search". A useful list of Modern Icelandic grammatical terms:
http://www2.hu-berlin.de/bragi/b3/tho/b3tho_17_malfraediheiti_fs.htm

Over the course of the middle ages, verbal inflections changed, so
that late medieval texts show many features of the modern paradigm,
e.g. the use of indicative forms in place of the subjunctive, and 3rd
person forms for the 1st person. The middle voice endings in
particular have gone through a lot of changes, sk > z > zt, zst,
before the modern 'st' became common in the 16th century. In the 1st
person of the middle voice, 'ek köllumk' very early appeared with the
same form as the plural, 'ek köllumsk', which was soon brought into
line with the rest of the singular: 'ek kallask', hence the modern 'ég
kallast'.

If this sounds daunting, don't worry. The texts we're using have been
normalised, whether to an early Old Icelandic standard (as in the case
of Gordon's text of Hrafnkels saga) or to a modern standard (as in the
case of this text of Njáls saga that we're looking at now). So the
textbook paradigms should be sufficient.

Okay, that might still seem like a lot of inflections to learn, but
it's not as bad as it looks. You can split up the task and
concentrate on learning these tables a little bit at a time. When you
know one section of the paradigm, the rest becomes easier as you start
to see the patterns. For the purpose of reading sagas, you don't need
to know the tables off by heart, just to be able to recognise verb
forms when you find them.

It can be a useful excercise to compare the old and modern standard
paradigms, and take note of where they differ. They're almost the
same, but note:

1st pl. subjunctive, kallim > köllum
1st sg. past indicative/subjunctive, kallaða > kallaði

And the past plural subjunctive forms kallaðim, kallaðið, kallaði have
now become the same as the indicative: kölluðum, kölluðuð, kölluðu.