In the first instance "Jim is being generous" means that being generous is something that Jim is doing at that moment. "Jim is generous" describes a character trait that Jim has. In the second example I would want the sentence to continue like "She was being harassed by the police when..." and the second is how we would state that something had happened without elaborating further.

Hi Victoria! In your first example "John is being generous" indicates that John is adopting the quality of generosity (present continuous - happening now and continuing into the future). It is usually used to imply an action that was not present in the past. Whereas "John is generous" implies that John is generous at this time (present simple - fixed in time). It is really a difference in verb tense. In the second example, it's the same as the example above except in the past. So, "she was being harassed by the police" is past continuous meaning that it started in the past and it's still going on (hasn't finished). It is useful when telling a story to give background eg. "I was eating when the phone rang". For the other one, "she was harassed" is simple past, indicates an event that happened in the past but stopped. Here is a good site which can help you with other english verb tenses http://www.edufind.com/english-grammar/verbs/