hair是不可数名词。不可以用定冠词a来修饰。可以说My hair is long。
I have a long hair---》我有一根长的头发My hair is long.---》我的头发很长我留着一头长发-----》I stay a head of long hair
hair不可数，前不能用a
有点怪，My hair is long.
把a 去掉即可
