잊다 means "to forget" Both 잊히다 and 잊혀지다 mean "to be forgotten" 잊히다 is passive verb of 잊다. 잊혀지다 is passive verb of 잊히다. 잊혀지다 is grammatically incorrect because it is double passive verb. But, 잊혀지다 is more commonly used than 잊히다 nowadays. e.g. 편지에 답장 쓰는 것을 잊었다. I forgot to answer the letter. 그녀가 잊히지 않았다 She was always on my mind. 그 사건은 사람들에게서 잊혀졌다 The event has been forgotten by the people.