nhw - they

maen nhw - they are

yfed - drinking/to drink

'n/yn - yn (in this usage) used as a particle to indicate an incomplete action, and not directly translated into English

/ Maen nhw / 'n / yfed/ -> / they are / [in the process of] / drinking / -> they are drinking

(for more about yn - https://www.duolingo.com/comment/13844144)