I’m sorry if this isn’t suitable, but I got to thinking and realised that I do not trust my memory to give me the plural of any word which I am not totally sure of!! (Age strikes again!). I was thinking ‘cath’ and ‘cathod’ popped up. I finally checked it and found it is right, but, in conversation, I wouldn’t have risked it!! Then I thought… how do you teach plurals? dyn, dynion; mochyn, moch, plentyn, plant, merch, merched, mynydd, mynyddyd, … and I can’t swear to the spelling of all those!! Are there any rules? And is the ‘es’ ending always female as in twysoges, brenhines, athroes etc.? I don’t remember how I learned them all!! (Well, was taught, I suppose!) 
In the book lessons it gets left til quite late, Uwch 2 probably. All they can do is describe the different forms and then we practice changing singles into plurals.
What we should do, but don’t, because there is so much more to do at the very beginning is learn both the gender and the plural of any noun we come across. I certainly didn’t.
A lot of animals (but not ci/cwn) pluralise with ‘od’
Brute force with memrise.com. Put in the singulars, put in the plurals, learn them together.
There are rules, but there are so many exceptions that you may as well not bother - brute force, as @margaretnock says 
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Just listen to lots of Radio Cymru, have lots of conversations and make lots of mistakes. That should do the job.
Also, bear in mind that some words are pluralised differently in different parts of Wales. This means that sometimes you can get a plural ‘wrong’ and have the person you’re talking to just assume that “that’s how you say it” in your part of Wales! 
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Clywch clywch! 
Diolch pawb!! I must have spent a lot of time learning them without remembering the process!! Ironically, the one time I’ve checked one before posting led to my use of ‘nicos’ for plural of ‘nico’ being questioned!!! 
It was this thread that made me look up and question nicos.
What’s that old song ‘ha ha said the clown’ (looks like i get today’s custard pie).
Cheers J.P.
With all the help you have given me, there is no way you deserve a pie except a really tasty one to eat!!! 
