Mmmmm. Ok, first off, as you know, it's not so simplified as English. But then if you think about it, for a learner of English making plurals isn't as easy as you think.
There are a few tips that I can give you (There will probably be exceptions!
The most productive plural is -au
Words ending in -ad
adroddiad - adroddiadau, syniad - syniadau
Words ending in -aeth
gwasanaeth - gwasanaethau
Words ending in -deb
undeb - undebau, diddordeb - diddordebau
Words ending in -es
(merch) athrawes - athrawesau
Words ending in -iant
methiant - methiannau, diwydiant - diwydiannau
The ending -aid
is added to some nouns for people and creatures:
cymar - cymheiriaid, estron - estroniaid, pagan - paganiaid
ffoadur - ffoaduriaid, pechadur - pechaduriaid, henadur - henaduriaid
anifail - anifeiliaid, blaidd - bleiddiaid, locust - locustiaid
The ending -od
is given to many animals:
llew - llewod, camel - camelod, cath - cathod
NB. Watch out! baban - babanod, gwrach - gwrachod, menyw - menywod
There are a couple other tips. I'll have a think.