In French, articles and determiners are required on almost every common noun; much more so than in English. They are inflected to agree in gender (masculine or feminine) and number (singular or plural) with the noun they determine, though most have only one plural form (for masculine and feminine). Many also often change form when the word that follows them begins with a vowel sound. [Source]
Où
sont
les
toilettes?
Where is the bathroom?
Parlez-
vous
l'
anglais?
Do you speak English? (Formal)
J'
ai
besoin
d'
un
médecin.
I need a doctor.