This is because when giving someone's name, occupation or relationship it is usual to emphasise that element in the sentence by putting it at the beginning:
- Ceri yw hi - She's Ceri
- Trydanydd yw Sioned - Sioned is an electrician
- Tad Ceri ydy Gareth - Gareth is Ceri's dad.
- Gareth sy'n dad Ceri, nid Geraint - It is Gareth who is Ceri's dad, not Geraint
In Welsh, we use use this change of word order for empasis more often than we do in English.