1. When Brian demands a straight answer from Wendy, he says:

Dyro atab strêt i fi, 'ta. - Give me a straight answer, then.

Welsh conversation, like English, is punctuated by seemingly meaningless tags, like 'ta, which corresponds to the English 'then'. Listen to some other examples:

Be sy'n digwydd 'ta? - What's happening, then?

'Nawr 'te - Now then.

'ta is the form used in North Wales. In the south, you'll hear 'te

The full form is 'ynte'.

2. Brian finds it very difficult to understand why Wendy has told him Frank was born in August, when in point of fact his date of birth is July.

Brian - Pam doeddet ti ddim yn onest efo fi o'r cychwyn? Pam dweud bod Frank wedi cael ei eni ym mis Awst, pan mai diwedd Gorffennaf ganwyd o.

Note the 2 ways Brian says 'was born':

(1) - wedi cael ei eni and (2) ganwyd.

Ganwyd is the past tense of the verb geni 'to be born'. Ganwyd doesn't change when referring to different persons.

Ganwyd fi - I was born.
Ganwyd fy merch - my daughter was born,
Ganwyd y plant - the children were born.

3. The Welsh way of saying 'if you'd prefer' is idiomatic and doesn't correspond to the English at all. Jenny says to Brian:

Os oes well gen ti... - if you'd prefer...

But, what the phrase means literally is: 'if it's better with you'.

Listen to some other examples of 'prefer' in different contexts

Mae'n well gen i goffi na sudd oren - I prefer coffee to orange juice

Mae'n well ganddi hi'r theatr na'r sinema - She prefers the theatre to the cinema.