Well I think you did well, all considered.

Amdanat ti mae son, Wennaf Wen, Wennaf Wen,
O Fynwy fawr i Fôn, Wennaf Wen;
I'r castell acw heno,
Rhaid iti droi a huno,
Hen deulu iawn sydd ynddo, Da ti mentra, mentra Gwen!

Of thee I speak, Fairest Gwen, Fairest Gwen,
From great Monmouth to Anglesey, Fairest Gwen.
To that castle over there tonight
You must turn and sleep,
The family living there is a good one, venture forth, venture forth, Gwen.

2. O'th flaen mae mynydd maith,
Wennaf Wen, Wennaf, Wen,
Gwell iti dorri'th daith
Wenaf Wen,
Wel yn fy mraich gan hynny,
Yr awn gan benderfynu,
Fod yn y castell lety;
Da ti mentra, mentra Gwen!

Before you the welcoming mountain, Fairest Gwen, Fairest Gwen
You had best break your journey, Fairest Gwen,
Wel, with this I shall take you on my arm
And we will go on having decided
That there are lodging to be had in the castle, venture forth, venture Gwen

3. Fi piau'r castell hwn,
Wennaf Wen, Wennaf Wen,
Ti elli fyw mi wn,
Wennaf Wen,
Y wraig yng Nghastell Crogen,
I'w barchu af a'i berchen;
A chymer fi'n y fargen,
Da ti mentra, mentra Gwen!

It is I who owns this castle, Fairest Gwen, Fairest GWen,
I know that you will be able to live, Fairest Gwen,
As wife of Castle Crogen,
To be respected by the owner
And taking me into the bargain,
Venture forth, Gwen, Venture Gwen.

(Oh lord! That's the Victorians for you!)

I have a feeling I never did PM you! There weren't many corrections to make. Here:

Y fwyn garedig ferch, mentra Gwen, mentra Gwen,
Lle rhoddais i fy serch, mentra Gwen;
Gwaith dyn wyf fi mewn galar,
yn ffaelu cerdded daear,
o'th gariad di'r fun hawddgar;
Rwy'n deisyf cael dy ffafar, mentra Gwen, mentra Gwen.

You seemed to have survived your mountain sojourn!

BTW, nice site (above). I commend to you particularly pretty tune: Maid of Llanwellyn. I've always been quite partial to it!

sian