Unable to remember when needed (but perhaps can at a later date). In this case, information is inaccessible
Memory simply forgets information, permanently, and the physical traces of the memory disappear. In which case, information is unavailable
No 'rehearsal'. The physical trace fades over time.
New memories replace old ones.
Memorise similar words. Can either be proactive (this is when old memories interfere with new ones) or retroactive, when new information distorts old memories.
Long term is supposed to be limitless in its capacity and length. Forget information through decay (as in short-term forgetting) and interference from other memories.