the definite articles in French corresponding to the English definite article THE are le, la & les.  Since you have been taught that all nouns have a specific gender the corresponding article must reflect that gender: For example the book = le (for masculine nouns book being masculine in French) livre. Or la (for feminine nouns) porte  (door). The definite article for plural nouns (masculine or feminine) is ‘les’.

The indefinite article in French corresponding to a/an (a building, an hospital) are un, une or des. Un livre =a book (rather than le livre which is the book); une porte= a door (rather than la porte which is the door) the plural ‘des’ indicates indefinite objects: des livres, des portes, des crayons.

So remember, a girl = une fille; the girl= la fille or an eraser= une gomme ; the eraser= la gomme. The plural for either is les filles or les gommes.