French Adjectives with Special Forms

Since French adjectives usually have to agree with the nouns they modify in gender and number, most of them have up to four forms (masculine singular, feminine singular, masculine plural, and feminine plural). But there are several French adjectives that have an additional variation: a special form that is used when the adjective precedes a word that begins with a vowel or mute H.
The reason for this special adjective form is to avoid hiatus (the pause between a word that ends in a vowel sound and another that begins with a vowel sound). The French language likes words that flow one into the next, so when an adjective that ends in a vowel sound would otherwise be followed by a word that begins with a vowel sound, French uses a special form of the adjective to avoid the undesirable hiatus. These special forms end in consonants so that an enchaînement is created between the two words, and the fluidity of the language is maintained.
There are nine French adjectives in three categories which have one of these special pre-vowel forms.

Descriptive Adjectives

The following descriptive adjectives have a special form that is used only in front of a masculine noun that begins with a vowel or mute H.

• beau > bel
  un beau garçon > un bel homme
  fou > fol
  un fou rire > un fol espoir
  mou > mol
  un mou refus > un mol abandon
  nouveau > nouvel
  un nouveau livre > un nouvel article
  vieux > vieil
  un vieux bâtiment > un vieil immeuble

Demonstrative Adjectives

When the demonstrative adjective is used with a masculine noun that begins with a vowel or mute H, it changes from ce to cet:

• ce garçon > cet homme

Possessive Adjectives

When a singular possessive adjective is used with a feminine noun that begins with a vowel or mute H, it changes from the feminine form (ma, ta, sa) to the masculine form (mon, ton, son):

• ma mère > mon amie
  ta femme > ton amante
  sa profession > son éducation

Note

The special adjective forms are used only when followed immediately by a word that begins with a vowel or mute H. If a word which begins with a consonant is placed between the changeable adjective and the noun, the special form is not used.
Compare:

• cet homme vs ce grand homme
• mon amie vs ma meilleure amie

When there's an adjective, the special form is not used because the word that immediately follows the changeable adjective begins with a consonant.