"Which hashtag are we using?"
Translation:Pa hashnod dyn ni'n ddefnyddio?
The base word nod can be masculine or feminine, so the compound noun can be either as well. This means that both ...yn (ei) defnyddio... and ... yn (ei) ddefnyddio... are acceptable.
(Groan!) Thanks - in a little while I may be able to think of these explanations myself. Possibly! Though it will take me longer to realise that "nod" is the root of "(d)defnyddio".
Ah... no, it is hashnod which is the compound noun of nod here. ei refers back to it, and so ei... can be either the masculine or feminine 'its...'.
ei (masculine) causes soft mutation (here, defnyddio - ddefnyddio), while ei (feminine) causes aspirate mutation (of p-, t-, c-) or adding h- to vowels, neither of which applies to defnyddio*.
If you want to keep your Welsh up in the future, it would be worth getting hold of a copy of something like 'Welsh Rules' by Heini Gruffudd. A good grammar book to be able to look things up in, and which covers the material in this course as well as going on into intermediate Welsh and the beginnings of more advanced Welsh. It is widely used in Welsh education.