Dear Mr. Alan. There is something that I don’t find very clear in your text on articles. I will copy two parts of it:
the first one
“Before a singular countable noun which is used as an example of a class of things.
«A book is something you read.»”
and the second one:
“Before singular nouns used to represent a class of objects.
«The donkey is a very obstinate animal.»”
ESL Lesson: Articles in English: The vs. A/an
I saw this kind of explanation in several textbooks but I’ve never understood it completely. Both examples seem to be the same, and still we use both definite and indefinite article.