一般情况在词尾加-s map-maps, sea-seas, girl-girls, day-days 2 以s, x, ch, sh结尾的名词后加-es class-classes, box-boxes, watch-watches, dish-dishes 3 以-f或-fe结尾的词 变-f和-fe为v再加-es leaf-leaves, thief-thieves, knife-knives, ...