When one says I was washing my hair, is it singular or plural? What is the singular for hair?
This seems to be one of those plural issues where a different plural is used when referring to the large uncountable group. "I found 3 gray hairs this morning" is proper but so is "I washed my hair this morning".
In the second case, your entire head covered with individuals hairs is treated as a single object or group which is why it is referred to in a singular form.
I think 'Hair' is Material noun. Brick, wood, skin, muscle, oil, glass, paper, paint, gold etc. are all Material Nouns. So, if wish to refer to a particular number of Hair, we should say 3 or 4 strands of hair.
Consider the sentences :
"The other was Della's hair". (not 'hairs')
My hair is turning grey. (not 'hairs are')
My hair is black and his hair is brown. (not 'hairs are')
He caught my wisp or lock of hair. (not 'hairs')
He plucked 10 strands of grey hair from my head. ('10 strands', but not 'hairs')
The word hair in some cases is a collective noun, and in other cases is not a collective noun.
As reported from the NOAD, the meaning of the word is:
In most of the phrases, the word used is hair; in some cases is hairs (to split hairs).
I think both hair and hairs are right because the sentence "I pulled out some hairs" would be right and "She has a head full of hair" works too, so I think both hair and hairs work that it just depends on the context clues of the sentence.
The noun hair is a singular, plural, or collective noun. It all depends on the context of the sentence.
In your sentence,
I was washing my hair.
the noun hair is the collective. It is also the case in:
I have my hair cut.
It is singular in following sentences.
I found a hair in my soup.
It is plural in the following sentences.
There are dog hairs on the sofa.
Neither, it is collective, meaning that 'hair' in this context refers to all your hair.
protected by Will Hunting Mar 22 '12 at 21:40
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