Grammar: Noun Phrase (Determiners and Quantifiers) - SS1 English Past Questions and Answers - page 1

The main difference between a definite and indefinite article in a noun phrase is that the definite article "the" is used to refer to a specific noun that has already been mentioned or is known to both the speaker and the listener, while the indefinite articles "a" and "an" are used to refer to a general or unspecified noun. For instance, "the book on the shelf" refers to a specific book that has already been mentioned or is known, while "a book on the shelf" refers to any book that is on the shelf.