Era and epoch are synonyms, however era refers to a period of time of indeterminate length of usually more than one year. 'The old houses in New Orleans were built during an era when craftsmen took pride in their work, therefore the houses are of high quality.' An epoch is a particular period of history usually in which especially noteworthy events took place. 'During the Cretaceous Epoch, though dinosaurs continued to dominate the land, new forms of mammals, birds and flowering plants began to appear.' In regard to your second question, I'm not quite sure what you mean by 'structure' of the sentence. It is a complete sentence (as opposed to a fragment or run on) and it is written in the present perfect tense.