"Here is a beautiful leaf."
Translation:Dyma ddeilen hardd.
As EllisV says, dyma is acting as the verb. You will occasionally see it described in grammar books as a 'demonstrative verb', along with dyna (there is...).
It is a contraction of a number of words which together used to mean something along the lines of 'look here'. You use it if you are pointing at something nearby or holding something with the purpose of drawing someone's attention to it.
Because dyma and dyna are derived from verbs, they both cause soft mutation of their object:
- Dyma dorth - Here is a loaf (torth)
- Dyna fynydd - There is a mountain (mynydd)
- Dyna ddyn sy'n edrych fel mae e'n gallu helpu ni - There is a man who look as though he can help us.