Englishfor English speakers
A is used when the following word could be any of a certain type.
Compare "A book I saw on the shelf" and "The book I gave you yesterday".
A is the first letter of the alphabet.
The letter "a" comes before "b".
In some schools, an A is a very high grade.
Ron got an A on his earth science test.
someone who is dazzlingly skilled in any field