On'd begins a negative question, e.g. on'd wyt? "aren't you?", on'd yw hi? "isn't she?", on'd oes? "isn't/aren't there?" etc. It was originally a combination of o "if" and ni(d) "not", giving you the fuller forms oni (before a consonant) and onid (before a vowel). Today, however, it's only really found in little tag questions at the end of phrases and is said very quickly as on'd, on' or even just 'd.
It is a different way of making a similar point, but it is not the same phrasing, I am afraid.
The mae .... gyda ti is - 'you have ...', whereas mae dy ... yn ... means 'your ... is ...':
- Mae gwallt golau gyda ti, on'd oes? - You have fair hair, haven't you?
- Mae dy wallt yn olau, on'd ydy e? - Your hair is fair, isn't it?
Dal ati! - Keep at it!
Better English would be , "haven't you". Would this be wrong as a translation?
Answering this on a second, seperate occasion, I gave 'haven't you' as an answer and was marked wrong. I strenuously object to the American 'don't' when 'haven't ' is better English. I suppose I'll just always get this wrong.
No. In this pattern the ...on'd oes? (...isn't there?) refers back to Mae... (There is...).
- Rwyt ti'n mynd i'r swyddfa heddiw, on'd wyt ti? - You're going to the office today, aren't you?
- Dych chi'n i'r gwaith heddiw, on'd dych chi? - You're going to work today, aren't you?
The 'on'd oes' in this phrase should be translated to, 'don' t you' not 'haven't you', i.e. 'you have fair hair, don't you?'
In English, the question tag is always based on the auxiliary verb used in the sentence. Since the sentence is in the present simple, the auxiliary verb would be the verb 'to do', not 'to have'.
'Haven't you' could be used if the sentence were in the present perfect, for example, 'you've seen his hair, haven't you?'. In this case, the auxiliary verb is 'to have', so it could be used in the question tag.