Brian's full of remorse that he's not kept his promise to Rhodri, who wants to live with him and Wendy full time, and he says:

Dw i heb gadw at fy ngair

I haven't kept my word or literally, 'I'm without keeping to my word'

heb means 'without' but it can be used to replace the negative word ddim which we hear all the time -

e.g. dw wedi clywed - I've heard

dydw i ddim wedi clywed - I haven't heard.

For Dydw i ddim wedi clywed you could say Dw i heb glywed.

Note also that heb has to be followed by the treiglad meddal - the soft mutation. Here are some other examples:

Dw i heb weld y ffilm eto - I haven't seen the film yet,

instead of

Dydw i ddim wedi gweld y ffilm eto.

Mae hi heb gyrraedd eto - She hasn't arrived yet

instead of

Dydy hi ddim wedi cyrraedd eto.

Michelle is smitten with Brian and tells him that if she were his girlfriend there'd be no problem having Rhodri to live with them.

Pe bawn i'n gariad neu'n bartner i ti, fyddai hynny ddim yn broblem o gwbl.

Pe bawn corresponds to the English 'if I were' or 'if I was'.

For example: Pe bawn i'n ti - If I were you.
Pe bawn i'n gyfoethog - If I were rich