All answers are fine, but there's something they forgot here, the termination "lo" that is used to tell is something YOU'RE saying, so it would look like this: "¿Cómo podría olvidarLO? and if your using past tense then it would be: "¿Cómo pude olvidarlo?, in english you would need to add "it" to the end of the phrase (How could I forget it?) But in spanish we usually don't need that, Hope this helps, anyway, as I said just the translation sounds like ¿Cómo podría/pude olvidar? ^^

¿Cómo puedo olvidarlo? Is like... Oh my dear, i love to play basketball, how could i forget? Success! Raff

HOW COULD I FORGET? ¿CÓMO PODRÍA OLVIDAR? HOW COULD I FORGET HER BIRTHDAY? ¿CÓMO PODRÍA OLVIDAR SU CUMPLEAÑOS? I HOPE IT BE USEFUL FOR YOU! greetings!*-*-*-*-*-*-*- Elian

Cómo podría olvidar? (potential) Cómo pude olvidar? (past) good luck!