Dear Nina,

> N: I wonder about the term vikara.na: vikaroti is to change. And non-
> vikara.na. Or do they mean: regular and irregular?

No, I don't think so. Although "vikara.na" isn't in the PED, you can find a
definition in Sanskrit dictionaries. Monier-Williams has the following:

vi-karaNa 2. m. (for 1. » [p= 950,1]) " producing a change. " (in gram.) a
term for the affix or conjugational characteristic which is placed between
the root and terminations , the inserted conjugational affix (according to
pANini these affixes are zap , zapo luk , zlu , zyan , znu , za , znam , u ,
znA , yak , and cli [with its substitutes] , tAsi , sya , sip , the first
nine of which are added in the Pres. , Impf. , Imperative , and Potential ,
and before a kRt which contains a mute palatal z , in the case of Active
verbs ; yak is added in the case of the karman or bhAva i.e. Passives or
Neuters ; cli is added in the Aorist , tAsi in the 1st Future , sya in the
2nd Future and Conditional , and sip before leT. ; lug-vikaraNa , " having
luk for its vikaraNa " [said of roots of cl. 2.] ; AkhyAta-pada-vikaraNAH ,
" words which modify the finite verb " i.e. make it accented)

So it has a wider application in Sanskrit than in Pali. The term is also
found in the Kaccaayana and Moggallaana schools. It seems that
Aggava.msa doesn't use the term in his Suttamaala, preferring to use just
"paccaya" instead.

Best wishes,
Jim