hair

hair统指“头发（或毛）”，不论说一个人或几个人的头发都该用hair，不该用复数形式。a hair指“一根头发”或“一根毛”，hairs指“多根头发”或“多根毛”。the hairs of my hair作“我的许多根头发”解，这语不普通。但grey hairs（老年、老年人们）里的hairs不可改作hair。hair和beard性质不同。一个人有一个beard，两个人有两个beards，如This old man has a long beard和These old men have long beards。但不论说一个人或几个人的头发都用hair，如This old man has white hair和These old men have white hair。This old man has a white hair是“这个老人有一根白发”。These old men have white hairs是“这些老人有白发”。