Class nouns are used with the indefinite article:
When the speaker presents the object expressed by the noun as belonging to a certain class.
She has a watch of her own.
On the green surface of the lake a little boat, with white wings faintly fluttering, rocked in the dewy breeze. (Voynich)
Close beside them grew a rose-bush covered with scarlet hips. (Voynich)
In the plural no article is used in this case. If the idea of number is implied the noun is preceded by the pronoun some.
Some, as well as the absence of articles with class nouns in the plural, is the equivalent of the indefinite article in the singular. Some is used when the speaker wants to emphasize the idea of number. Some is also used with nouns of material if the idea of quantity is implied. Some has the meaning of ‘several’ with class nouns and ‘a little’ with nouns of material.
I liked the room because there were flowers in it.
“I have brought you some flowers…” “I hate to wear flowers.” (Voynich)
With a predicative noun, when the speaker states that the object denoted by the noun belongs to a certain class.
Miss Sharp’s father was an artist. (Thackeray)
“Is your brother an agreeable man, Peggotty?” “Oh, what an agreeable man he is!” (Dickens)
She works as a chemist. (Cronin)
In the plural neither the article nor the pronoun some is used.
After the conjunction as a predicative noun is often used without an article.
She was engaged as governess.
When the noun is used in a general sense. What is said of one representative of a class can be applied to all the representatives of the class. The article has the meaning of ‘every’.
A drowning man catches at a straw.
In the plural neither the article nor the pronoun some is used. Real friends should have everything in common. (Wilde)
There are cases when the indefinite article preserves its old original meaning of ‘one’.
A stitch in time saves nine.
He had hardly spoken a word since they left Riccardo’s door… (Voynich)
This meaning is generally found with nouns denoting time, measure and weight.
A week or two passed. (Ch. Bronte)
“I’ll overtake you in a minute,” said Godfrey. (Eliot)
With the numerals hundred, thousand, million and the nouns dozen, score.
He seems to have half a dozen languages at his finger-tips. (Voynich)