Honestly, plurals are a bugger - there are so many rules that you just can't memorise them. You really should learn them as you go. A few plurals are distincitive though, and you'll start to see patterns, but I really can't get into a comprehensive discussion of plurals without a massive great big post which, frankly, won't help you because by the time you've got to the end of it your brain will have melted. I know mine will if I try and write it. ;-) Get Gareth King's Modern Welsh and take a good look at his section on plurals - it'll explain it better than I can. In fact, I'd only be copying it, so best get it from the horse's mouth. Oh and btw, not being a native speaker i'm not the best person to ask for advice on pronunciation, which is probably why i delayed responding to your earlier comment asking about 'Gwyl' which I pronounce something like 'gweeyl', but i might be saying it wrong. ;-)