"The vodka is three pounds."
Translation:Tair punt yw'r fodca.
(Sorry, only having reread my first answer do I realise that it doesn't make any sense!)
Both yw and ydy come from the older form which incorporates them both, which is "ydyw". Over time this one word has become two, and which one is used will depend on the speaker. Generally, yw is used in South Wales and ydy in North :)