Understanding French Numerical Adjectives

In the  French language, numerical adjectives aren't as complicated as they might sound - numerical adjective is simply the grammatical term for numbers. There are three types of numerical adjectives, each used for a different purpose - see table at the bottom of the page.

Placement of Numerical Adjectives

Cardinal numerical adjectives precede the noun they modify, as well as any other adjectives (numerical or not) that precede the noun.

   J'ai deux livres.
   I have two books.

   Il a acheté une nouvelle voiture.
   He bought a new car.

   les trois premiers jours
   the first three days

Ordinal numerical adjectives, multiplicatives, and the fraction demi usually precede the noun they modify:

   C'est le deuxième jour.
   It's the second day.

   Il veut un double whisky.
   He wants a double whiskey.

   J'y vais dans une heure et demie.
   I'm going in an hour and a half.

Fractions, other than demi, require the following format in front of nouns: article/number + fraction + de:

   J'ai regardé un tiers du film.
   I watched a third of the movie.

   Il a bu deux cinquièmes de la bouteille.
   He drank two fifths of the bottle.

 Agreement of Numerical Adjectives 

Only a few numerical adjectives agree with the nouns they modify.

1. Cardinal numbers - all invariable, except for "one":

   un homme (one man) / une femme (one woman)
     vs
   deux hommes (two men) / deux femmes (two women)

2. Ordinal numbers - "First" is variable. The rest are invariable, but note that if preceded by a definite article, it must match the gender of the noun:

   le premier livre (the first book) / la première peinture (the first painting)
     vs
   le deuxième livre (the second book) / la troisième bouteille (the third bottle)

3. Multiplicative numbers are all invariable.
4. Fractions - demi can be masculine or feminine, while the others can be singular or plural:

   un demi kilo (half a kilo) / une demie bouteille (half a bottle)
     vs
   un quart (one fourth) / trois quarts (three fourths)

Types of Numerical Adjectives

*Fractions, with the exception of demi, are nouns rather than adjectives, but it makes sense to include them with the other types of numbers.

Technically, numerical adjectives are not adjectives at all - they are a mathematical feature which, grammatically, act more or less like adjectives.