> Thank you for your answer. I have not studied Old Norse at any university
> so I have not learned etymology. Is there any short and simple rule when
> to use the nasal "a" when writing runes?

No :)

> But why are the Old Norse long nasal vowel phonemes not
> marked in the normalized orthography? Are they not considered "important"
> enough,or is it just too uncertain when they were used?

Your question is good and so are your suggested answers.
It *is* true that we don't know enough about them to mark
them with confidence. Such marking would also be difficult
to print. It also wouldn't really help any readers since
the nasal vowels have long since merged with their non-nasal
counterparts in Icelandic pronunciation.

One further reason is that when the standardized orthography
for Old Norse was developed the presence of the nasal vowels
in the language was still not accepted by philologists, despite
the clear testimony of the FGT.

Kveðja,
Haukur