Hair为不可数名词的时候用is。hair这个词既可以是可数名词，也可以是不可数名词，只是含义稍有变化。hair作为不可数名词是“头发”的总称，是集合名词。hair作为可数名词，是指一根一根的“头发”或“毛发”。
hair的例句
She walked forward and embraced him and stroked his tousled white hair.
她走上前拥抱他并轻抚他凌乱的白发。
Applying the dye can be messy, particularly on long hair.
涂抹染发剂可能会搞得一团糟，特别是长发。
His long, uncovered hair flew back in the wind.
他那露在外面的长发随风向后飞舞。
He was a tall, thin man with grey hair.
他是个瘦高个，头发灰白。
He had long unkempt hair and a stubbly chin.
他的头发又长又乱，脸上胡子拉碴。
His hair was disheveled, and his face was unshaven and gray.
他头发凌乱，没刮胡子，脸色苍白。
Sarah lifted her hand and started twirling a strand of hair.
萨拉抬起手来开始用手指缠绕一缕头发。
Teddy ran thick fingers through his unruly thatch of hair.
特迪用自己的粗手指头捋了一下浓密蓬乱的头发。
Her cousin, a hairdresser, was perming her hair as a special treat.
她表兄是美发师，正在免费给她烫发。
She unbound her hair and let it flow loose in the wind.
她把头发解开，让它随风飘动。
