"Mae'r tân yn berwi'r dŵr ac mae'r dŵr yn stemio."

Translation: The fire is boiling the water, and the water is steaming.

[ChrisMSully]

Pam 'ac', dim 'a' o'r blaen 'mae'?

[ibisc]

Well spotted! ac instead of a because of the ghost of the particle y in front of mae. (The y there is not the 'r/y/yr you have seen so far) In more formal Welsh you would usually use y mae instead of just mae. In colloquial Welsh the y is dropped.

Some common places where ac is used for various reasons - ac na, ac mor, ac felly, ac fel, ac mae, ac maen, ac sy, ac roedd, ac mewn, ac fe/mi.