"What are you able to do?"
Translation:Beth dych chi'n gallu wneud?
Could you please add Beth dach...? (And perhaps also Be' dach... which is, I believe, the form taught by WJEC Mynediad North.)
The verb 'gallu' is followed by the inifinitive of a verb
Thus:- 'to be able to sing' = gallu canu; 'to be able to run' = gallu rhedeg....etc.
However this sentence combines two different constructions:-
To do = gwneud
To do it = 'ei wneud' (ei = it;- literally 'to it do')
You are able to do it = dych chi gallu'n ei wneud
Putting 'beth' at the beginning of the statement turns it into a question.
What are you able to do (it) (lit:- What you are able to do it) = Beth dych chi'n gallu (ei) wneud.
In general speech the pronoun gets dropped which gives the above.
(You are able to do it = dych chi'N gallu ei wneud -- needs an 'n in it, I think. Feel free to delete this comment.)
Soft mutation after ei -- ei wneud "to do it" or literally "its doing", i.e. "What are you able its doing = What can you do?"
The ei often gets dropped but the mutation remains. See the comment by rmcode in response to keentospeak.