Good questions, Gael. No need to apologise! These masculine nouns with
u-mutation originally had an unstressed /u/ in some of their endings,
including the nominative and accusative singular (and the accusative
and dative plural). This /u/ is sometimes called the stem vowel or the
thematic vowel. By "originally" I mean in an earlier stage of the
language, before the Viking Age.

*skelduz > *skealduz > ON skjöldr "shield"
*arnuz > ON örn "eagle"

(That asterisk * indicates hypothetical reconstructions.)

The /u/ in this class of nouns was inherited from Proto-Germanic and
can still be seen in Gothic (Go. skildus, ON skjöldr), and in Old
English where the root syllable is short (Go. magus, OE magu, ON mögr
"son, boy"). It isn't related to the epenthetic /u/ of Modern
Icelandic which was a much later development that affected all noun
kinds of nouns that ended in a consonant followed by /r/. The
epenthetic /u/ didn't cause u-mutation.

These masculine "u-stem" nouns, as they're called, are masculine in
related old Germanic languages, and probably also in Proto-Norse and
Proto-Germanic. There are a very small number of feminine u-stem nouns
in Gothic and Old English, e.g. Go. handus, OE hand "hand". Probably
ON hönd "hand" once followed the u-stem declension, but the only trace
of this left is the dative singular: hendi.

The u-stems are relatively few compared to those masculine nouns that
originally had other vowels in those endings. That's why most
masculine nouns don't have u-mutation. Of masculine nouns ending in -r
in Old Norse, the most common type had /a/ in the nominative and
accusative singular endings in Proto-Germanic and Proto-Norse.

*dagaz > ON dagr (Go. dags, OE dæg).

Others had /i/ for their stem vowel, which sometimes results in
i-mutation in the Old Norse word. But often unmutated forms appear,
especially where the root is short:

*balgiz > ON belgr "skin; skin bag; bellows" (Go. balgs, OE belig).
*stadiz > ON staðr "place" (Go. staþs, OE stede).

...and a few more classes too, each with their own peculiarities;
likewise for neuters and feminines. Hope that helps. Let me know if I
lapsed into jargon or if there's anything else you're unclear about.

LN



--- In norse_course@yahoogroups.com, "Gaël DEEST" <gael.deest@...> wrote:
>
> Hello,
>
> I'm trying to refine my understanding of ON by studying inflexions
> with a more phonological approach than I did before. This lead me to
> the following question (please excuse the amateur I am if it is stupid
> !) :
>
> I remarked that some nouns (eg. skjǫldr, ǫrn) are subject to
> u-mutations in the singular nominative and accusative, much like in
> the strong neuter plural, but it is not always the case (eg. staðr).
> Can it be explained in more details ? Were these nouns feminines at an
> earlier stage of the language ? Does this have anything to do with the
> "epenthetic u" of modern icelandic ? (maðr vs. maður)
>
> Thanks !
>
> Gaël
>