Definite and Indefinite Articles
There is a clear difference between the definite and the indefinite articles.
Indefinite article: Do you have a book? / ¿Tienes un libro? Yes, I have a book. / Sí, tengo un libro. Which book do you have? / ¿Cuál libro tienes? I have Harry Potter and the ..... Tengo Harry Potter y ....
Definite article: Do you have the book? / ¿Tienes el libro? Which book? / ¿Cuál libro? The Spanish book. / El libro de español. / Yes, I have the Spanish book. / Sí, tengo el libro de español.
Quite simply, Harry Potter stands for <<any old book>> unspecific or indefinite, while the Spanish book is just that, the specific book we use in this specific class.
We have already met the indefinite articles (a, an, some) and they can be masculine, feminine, singular and plural. Note that the French forms un, une are supplemented in the plural quelques livres, or, without the noun, quelques-uns, quelques-unes)
Singular Plural Masculine un libro unos libros Femine una mujer unas mujeres Remember that unos / unas means some in Spanish.
The definite articles (the -- unchangeable in English, but changeable in French -- le, la, l', les) operate in a similar manner.
Singular Plural Masculine el libro los libros Femine la mujer las mujeres Remember that gender (masculine or feminine) and number (singular or plural) is determined by the noun and that articles -- definite or indefinite -- must agree with that noun. If a combination of masculine and feminine nouns tis used, then the masculine takes precedence in Spanish.
Remember too that the indefinite article apocopates before a masculine singular noun: Do you have a book? / ¿Tienes un libro? Yes, I have a book. / Sí, tengo un libro. BUT: Yes, I have one (no noun). / Sí, tengo uno. Remember, too, the more difficult concept: 21 houses / veintiuna casas (twenty houses and one house); 21 pencils / veintiún lápices (twenty pencils and one pencil).