These are conjunctive pronouns. Here is a full list:
minnau |
tithau |
yntau |
hithau |
ninnau |
chwithau/chithau |
hwythau |
Conjunctive pronouns are used:
to compare, e.g.
Bwytais i frechdan i ginio ac yntau sglodion.
I ate a sandwich and he ate chips.
as a link to the end of the sentence or paragraph, e.g.
 ninnau’n mwynhau ar y traeth, daeth y glaw.
As we were enjoying on the beach, the rain came.
to express ‘also’ or ‘too’ (as in this example), e.g.
Dylent hwythau (hefyd) wybod yn well!
They (also) should know better.