"Ble mae'r ddesg talu?"
Translation:Where is the checkout?
y ddesg talu - 'the desk of paying' => the payment desk/checkout
y ddesg dalu - 'the paying desk' ('paying' used as an adjective) - possibly a checkout, but possibly also somewhere with lots of money to give away...
Hm, tricky stuff. I think I see what you're saying - but then how about Cymanfa ganu? Isn't that a festival of singing, not a "singing festival", i.e. the festival itself isn't singing, just as the desk isn't actually paying?
Tricky indeed! According to y Termiadur Addysg, a standard reference for Welsh, it is definitely desg talu.
With cymanfa ganu it is the cymanfa, the assembled people, who are doing the singing.
But I shall ask an expert during the week...