"There is so much work left."
Translation:Mae cymaint o waith ar ôl.
No, you couldn't use 'dyna' in this sentence, you would use 'dyna' for 'there is' in a phrase like 'dyna fe!' - 'there he/it is' (lit there is he/it) or 'Dyna Megan' 'There's Megan'.
Mae without a pronoun or proper noun means 'there is/are'.
Can I repeat the first query here? Why are 'cymaint' and 'gymaint' both correct, or is it just a common usage thing?
I wonder if it is because up north we use "mae 'na" for "there is" and this takes a mutation. I presume therefore that correctly it would be "mae 'na gymaint" (N) and "mae cymaint" (S) and the "mae gymaint" has become so common that it is accepted but perhaps not felt to be quite correct?