Prynhawn da - Good afternoon
Noswaith dda - Good evening
Nos da - Good night
Shw mae - Hello
Sut da chi? - How are you (not sure about spelling!)
Da iawn - very well
Diolch - thank you
Hwyl - bye!
pob lwc - good luck
Penblwydd hapus - happy birthday (often to be seen on the happy birthday threads

Dim problem - Anglicised verion of no problem

wela i ti wedyn - See you later
Ych y fi - YUK!
duw duw - well,well. (literally translates to 'god,god')
ardderchog - Excellent
twll din - ar*e holes

Rhedeg - Run
Rhedwr - runner
Wedi blino - tired (useful after a Marathon)
Marathon - Marathon. Only in Welsh the word Marathon comes before the name (e.g. Snowdonia Marathon is Marathon Eryri)
Anaf - Injury
Ymofyn - Fetch
Ymofynwr - Fetchie
Ych a fi - yeuch! (no idea of the spelling) oh hang on we had that one.
Trygerethr - junk (no idea of spelling)
Blwyddyn newydd dda - happy new year (totally rubbish spelling)
Snwcer - snooker (I might have spelled that one correctly)
Bara menyn - bread and butter (crap spelling probably) no, correct spelling
Twp - stupid
Cerdded - To Walk
Ymddeol - To Retire
diog - lazy
cysgu - sleep
rhech- pop off
Ty Bach- little room (toilet)
Twll dyn pawb y fobyn - Go stick it up your arse