”I bought a book yesterday.”
”A man knocked on my door yesterday.”
I have been taught that the rules of ”a” and ”an” are to introduce something to the reader or listener, or to refer to any non-particular member of a group.
Are the indefinite articles doing these two actions at the same time? Or are there occassions when ”a” and ”an” only refer to any non-particular member of a group and does not introduce something and vice versa?
