"Rhaid iddyn nhw fynd."

Translation: They must go.

[jenna683537]

What is the 'iddyn' for in this sentence? Why isnt it 'rhaid i nhw fynd?'

[ibisc]

This is explained in the notes for this section. See https://www.duolingo.com/skill/cy/MustMustNot/tips-and-notes

Several prepositions in Welsh conjugate for person, and i is one of those. The form of i changes depending on the following pronoun - i mi, i ti, iddo fe, etc - and the full table for i is in the notes