I thought I had sorted wedi in my head. I understand Dw i wedi talu for I 've paid but what if I want to say ‘have I paid’ is Ydw i wedi talu? incorrect because on dysgwr website it states should be Dw i’n talu?.
You are spot on with ‘Ydw i wedi talu?’ meaning ‘Have I paid’. 
‘Dw i’n talu’ is in the present tense - ‘I am paying’, and ‘Am I paying?’ would be ‘Ydw i’n talu?’
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Dr Gramadeg dives in… http://parallel.cymru/?page_id=3452&lang=en
Wedi is the perfect tense. You can think of wedi as meaning have or has here. It is the same as the present tense but wedi is substituted for yn, e.g:
Dw i’n cyrraedd > I am arriving / I arrive
Dw i wedi cyrraedd > I have arrived
Wedi and yn never appear next to each other.
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