The soft matter covering your head is usually referred to using a plural noun in other languages, e.g. die Haare in German or les cheveux in French. In English, however, “hair” is a mass noun (just like “fur” or “grass”), and as such it is used with singular verbs:
Since it is a mass noun, we don’t use the indefinite article before it:
It should be noted that “hair” in English does not refer only to the hair between one’s forehead and nape; it refers to any kind of “fur” covering some part of a person’s body. To avoid possible confusion, we often use adjectives when referring to “non-head” hair, e.g. facial hair (the beard and mustache in men), pubic hair (hair around the genital area), body hair, and so on.
Hair can also be used as a countable noun when referring to a single strand of hair. For example, you can say:
You can also use “hair” in the plural when referring to several isolated hairs:
However, this is a relatively rare usage. When you speak about hair covering some part of the human body, you should never use “hairs”: