Singular vs Plural – Is ‘Staff’ Singular or Plural?

Whilst a dictionary search shows that the word is singular but plural in form, it is most commonly used (in my experience) with a verb in singular form.

Math(ematic)s is my least favourite subject.

Math(ematic)s is really hard.

This Ngram viewer helps:

Link to Ngram

EDIT: As @Araucaria points out, the "mathematics are" examples in the Ngram above are not really relevant. See his comment below.

Hair is both countable and uncountable Noun, but it is usually singular when it refers to all the hairs on one's head.

Example:

George has brown hair.

But if it refers to more than one hair, a few hairs, then it takes the plural form and needs a plural verb.

Example:

George has brown hair, but I found a hair on the sofa and suspect he's getting some gray hairs.

When you are talking about specific strands of hair, use the plural form.

Simply put:

Hair can be singular (one hair)

Example:

I found a strand of hair on your sofa. or I found a hair on your sofa

Non-countable singular (when it refers to all the hairs on one's head)

Example:

Shawn has black hair.

Or plural (three hairs, some hairs)

Example:

I Found not one, but three hairs on your sofa.

As Maulik.V said,"To make 'hair' singular, you need to quantify it. So, 'I found a strand of hair on the sofa.'"

• A strand of hair = One single hair
• Strands of hair = two or more, it does not specify

Note that we do not say "Strands of Hairs."

Thanks Maulik.V and Snailboat.