The verb oes comes from the verb-noun bod (being, to be). It is a special form of the third person singular (he, she, it is) which is used to ask questions about the existence of things, and to give the initial answer to the question:
- Oes car? Oes. Nac oes. - Is there a car? Yes, there is. No, there isn't
- Oes trên o Aberhonddu i Gaerdydd? Nac oes - Is there a train from Brecon to Cardiff? No, there isn't.
- Oes cath gyda hi? Oes, mae cath gyda hi. - Does she have a cat? Yes, she does have a cat.
As is often the case, the whole structure of the Welsh sentence is different from the English one here. In the Welsh sentence, the subject is cath (cat), so we know from the start what it is that we are talking about; who owns it comes next in the sentence, so it is not too long to wait...
It is just a matter of practice and recognising the meaning in the patterns - this will come in time. A word for word translation between Welsh and English is often not very useful.