Beic is a masculine word and therefore adjectives do not mutate softly after it (so y beic glas) whereas sgert is a feminine noun and therefore adjectives do mutate softly after it (so y sgert las). Another example of this is the phrases "Bore da" and "Nos dda". Since "bore" is masculine you have a "da" morning whereas "nos" is feminine and so you have a "dda" night.
All of the soft Mutations are as follows:
T - D
C - G
P - B
D - Dd
G - / (the g is lost)
B - F
Rh - R
M - F
Ll - L
I remember these letters using T, C, P Doctors Go Bananas, Rhian Marries Llywelyn.
Thank you, Ellis. Now all I have to do is find out which nouns are male and which are female. Just had a thought: There aren't neutral ones as well, are there?
Nope Welsh has no neuter gender, but some nouns can be treated as both genders, typically depending on dialect. (one that springs to mind is Tafarn(Pub) which you will see as "y dafarn" and "y tafarn" with both being considered correct.)
Did this sound like "Dw i eisiau'r sgert las" to anyone else? Cause.... I got it wrong :'(