"What has been stolen?"
Translation:Beth sy wedi cael ei ddwyn?
Is it because "Beth" is the subject rather than the object of the core sentence? "X has been stolen" > "What has been stolen?" (vs. perhaps something like "The thief has stolen X" > "What has the thief stole").
Would "What did the thief steal" be "Beth yw'r lleidr wedi dwyn?"
One literal translation could be 'What is that has had its stealing'?
This is transposed to 'What has been stolen?'
This is a passive sentence the stealing has happened to the 'what' , as already mentioned this is the subject of the verb.
Normally 'beth' is the interrogative 'what'
So a simple sentence would be 'Beth mae'r lleidr wedi ei ddwyn? - What has the thief stolen (it)?