"Did I understand what they were talking about?"
Translation:Ddeallais i am beth o'n nhw'n siarad?
It is not a matter of 'affirmative' or not. In many simple questions beth...? should be followed by ydy/yw, oedd, fydd, etc, but in this case the question is actually am beth...?, and this is followed by mae, roedd, bydd, etc (similarly with any case where a preposition precedes the beth or pwy as part of the question, as well as in certain other cases).
However, the r- of roedd, etc is often dropped in speech especially, so ...o'n nhw... is accepted here as well as the 'correct' ...r'on nhw....
It can get a little complicated to decide between the verb-forms to use after pwy and beth - don't worry about it too much as your meaning will always be understood.
I see. This question (which verb form to use after question words) is indeed one of the more difficult ones to get right… Does the phenomenon only happen if beth/pwy is preceded by am or is this a general rule valid for all prepositions? For example:
- I bwy (ydy/mae)’n rhaid dysgu hwnna?
- Gyda phwy (ydy/mae) e’n mynd ar wyliau?
- Ar feth (ydy/mae) hi’n gwrando?
- At bwy (ydy/mae)’r llythyr?
- Yn erbyn beth (ydyn/maen) nhw’n protestio?