The word 'the' is known as the definite article. In French it changes depending on the word it goes with - for masculine words it's le, and for feminine words it's la.
If a word starts with a vowel or with the letter -h, le and la both shorten to l', whether it's masculine or feminine.
L'Arc (m) de Triomphe L'Hôtel (m) de Nice L'aéroport (m) de Roissy L'homme (m)
If a word is plural,
le,
la and
l' all become
les. An -s is added to the end of plural words, though usually it's only pronounced when it comes before a vowel:
le taxi
les taxis
la rue
les rues
l'arc
les arcs
Unlike in English, you also use the definite articles when talking about things or people in general:
Les Françaises J'aime le café Les taxissont chers
French women
I like coffee
Taxis are expensive
Generally, articles are more frequently used in French than in English - you'll rarely hear a noun without one of them.