"It was hailing Monday."
Translation:Roedd hi'n bwrw cesair dydd Llun.
I'd qualify that and say some mutations are required. Certain mutations are essential to the meaning of a sentence or just sound jarring to the ear if forgotten. Others are pedantic and more or less obsolete. Many lie in between those two points. As a beginner the best thing to do is learn the ones presented in your course and later on as you develop more frequency you can learn to vary your mutations according to the situation.