Most French nouns become feminine according to regular patterns, but there are a number of irregular nouns, based on the final letter(s) of the masculine singular noun.

Nouns that end in a vowel plus L, N, or T usually become feminine by doubling the consonant before adding E.

Ending: en > enne   Noun: le gardien (guard)
   Masculine singular   le gardien
   Feminine singular   la gardienne
   Masculine plural   les gardiens
   Feminine plural   les gardiennes

Ending: el > elle   Noun: le colonel (colonel)
   Masculine singular   le colonel
   Feminine singular   la colonelle
   Masculine plural   les colonels
   Feminine plural   les colonelles

Nouns that end in er need a grave accent:

Ending: er > ère   Noun: le boulanger (baker)
   Masculine singular   le boulanger
   Feminine singular   la boulangère
   Masculine plural   les boulangers
   Feminine plural   les boulangères

The final letters eur have two possible irregular feminine endings:

Ending: eur > euse   Noun: un danseur (dancer)
   Masculine singular   un danseur
   Feminine singular   une danseuse
   Masculine plural   des danseurs
   Feminine plural   des danseuses

Ending: eur > rice   Noun: un acteur (actor)
   Masculine singular   un acteur
   Feminine singular   une actrice
   Masculine plural   des acteurs
   Feminine plural   des actrices

Notes

• These rules are similar for making adjectives feminine
   
• The rules for making nouns feminine apply only to people and some animals. They do not apply to objects, which have only one form: masculine or feminine.
   
• Compound nouns have their own gender rules.

French noun gender quiz