Translation:You brought a partner to the party on Saturday night.
Good question! In Welsh you have the option to emphasise part of a sentence by putting it at the front before the verb. When you do this with the subject (the bit that comes straight the verb in a normal sentence - here, chi a phartner), the verb reverts back to its "neutral" form, which is always the third person singular, whatever now comes before it. There's also a soft mutation on the verb.
Daeth Marian i mewn i'r siop "Marian came into the shop" (non-emphatic)
Marian ddaeth i mewn i'r siop "Marian came into the shop" (emphatic)
Daethon ni nos Wener "We came on Friday night"
Ni ddaeth nos Wener "We came on Friday night"
Dest ti heb ddigon o arian "You came without enough money"
Ti ddaeth heb ddigon o arian "You came without enough money"
Daethoch chi a'r plant "You can the kids came"
Chi a'r plant ddaeth "You and the kids came"