Is it possible to use "mae" here? I.e. "Mae'r cyfrifiadur a'r bysellfwrdd caledwedd." Surely this can be seen as a descriptive sentence as much as an identification sentence (to use terminology similar to that of Gareth King's book).
Thanks for the quick reply. That makes sense. I knew you needed "yn" for a verb-noun or an adjective but forgot you needed it for regular nouns too. Thinking about it, you must since grammatically speaking verb-nouns are nothing but nouns themselves.