There are no absolute rules for using double n in Welsh. By now, spellcheckers such as Cysill, have made life a little easier. However, here are a few rules (and their exceptions) which may help:
There are two ns after an -
an + t = annh e.g. an + tebyg = annhebyg
an + d = ann e.g. an + dymunol = annymunol
an + n = ann e.g. an + naturiol = annaturiol
BUT
an + tr =anhr e.g. an + trefn = anhrefn.
There are two ns when we add a syllable to nouns ending in –nt, e.g.
dant – dannedd
cant – cannoedd
llwyddiant – llwyddiannau
peiriant – peiriannau
diwylliant – diwylliannol.
There are two ns when we add a syllable to nouns ending in -en or –yn, e.g.
llythyren – llythrennau
llyfryn – llyfrynnau
dogfen – dogfennau
offeryn – offerynnau
BUT
emyn – emynau
gwenyn – gwenynen
telyn – telynau.
When we add a syllable to words ending in –iwn , the n doesn’t double, e.g.
cwestiwn – cwestiynau, cwestiynu
ffasiwn – ffasiynau, ffasiynol
fersiwn – fersiynau.
The n doesn’t double in front of -wyr, -ion, -iad, e.g.
diflannu – diflaniad
offerynnau – offerynwyr
ysgrifennu – ysgrifenwyr
cyfrannu – cyfraniad
gwynnu – gwynion.