Brian maintains that Jac's gone out of his way to spite him
Brian - Mae Jac wedi mynd ati i wneud hyn yn fwriadol. Er mwyn fy sbeitio fi.

You've already learnt that 'mynd at' means 'to go to a person or place'
For example: 'Dw i'n mynd at y meddyg' - 'I'm going to the doctor's.'
But used in this context, 'mynd ati' means 'to set out to do something'.
So again, 'Mae Jac wedi mynd ati i wneud hyn yn fwriadol.'
Also note that it's the feminine form of at, namely ati, which is used in this sense.
Tell your friends they should 'mynd ati i wrando ar Ysbyty Brynaber'.

You'll hear that Agnes is a member of Merched y Wawr, which is the Welsh equivalent of the Women's Institute. All activities are conducted through the medium of Welsh. They also welcome learners who'd like to improve their Welsh by socialising with native speakers in a natural situation. But Merched y Wawr do more than just socialise. Their activities range from learning a craft to going on a trip to somewhere of interest. They also support charities and promote women's causes, as well as the language and culture of Wales.

The form 'leiciet ti?', as a colloquial alternative to 'hoffet ti?', has been discussed in earlier editions of Ysbyty Brynaber, as in the example 'Beth leiciet ti?' What would you like?'. But in one of this weeks' episodes you'll hear Michelle asking Brian, not if he'd like anything, but if he'd like her to do something.

She says 'Leiciet ti i fi helpu?' - Would you like me to help? In this sense, leiciet ti must be followed by the preposition 'i'. Leiciet ti i fi helpu? The literal translation is 'Would you like for me to help?'

Here are some other examples:
Leiciet ti i mi fynd - Would you like me to go?
Leiciet ti i mi dalu? - Would you like me to pay?