Dw i'n mynd i'r llyfrgell
When saying "I am going to [insert place here]" how come some words have a soft mutation and some don't? For example, "I am going to the pub" causes tafarn to go to dafarn, but saying "I am going to the library" i.e. "Dw i'n mynd i'r llyfrgell" doesn't cause llyfrgell to change to lyfrgell? Can someone please explain this to me.
The prepoistion i (to/for) causes a soft mutation of a following word. If it is i'r... (to the...), then the 'r 'catches' the mutation. However, 'r/y will cause its own weak soft mutation of a following feminine noun, so tafarn - y dafarn - i'r dafarn.
In a weak soft mutation there is no mutation of ll- or rh-, so even though llyfrgell is a feminine noun it resists the mutation by 'r/y, so llyfrgell - y llyfrgell - i'r llyfrgell.
Some info and examples here: https://www.duolingo.com/skill/cy/The-1/tips-and-notes