"It was too wet yesterday."
Translation:Roedd hi'n rhy wlyb ddoe.
So I take it "rh" doesn't take soft mutation after "yn" as a sentence connector?
Correct, as in the examples shown in the 'Hints and Tips' for this section. I have just added an extra sentence to explain it a little more - ll- and rh- resist mutation by yn in any of its roles.
yn, y (the), un and mor (when it means 'as, so') all cause 'weak' soft mutation - ll- and rh- resist the mutation.