Formal:
* J'ai oublié. (pronounced "zhay oo-blee-ay") - This is the most common and direct way to say "I forgot".
Informal:
* J'ai zappé. (pronounced "zhay za-pay") - This is a more colloquial expression meaning "I forgot".
* J'ai oublié de. (pronounced "zhay oo-blee-ay duh") - This means "I forgot to..." and is followed by the action you forgot to do.
* Ça m'est sorti de la tête. (pronounced "sa may sor-tee duh la tet") - This means "It slipped my mind" or "I forgot about it".
Examples:
* J'ai oublié mon téléphone. (I forgot my phone.)
* J'ai zappé de te dire bonjour. (I forgot to say hello to you.)
* J'ai oublié de prendre un parapluie. (I forgot to take an umbrella.)
* Ça m'est sorti de la tête de t'appeler. (I forgot to call you.)
Remember to choose the appropriate expression based on the situation and the person you are speaking to.