Short answers, pulled from the story.
The Roman Empire is traditionally dated from 27 BC, when the Senate granted Octavian the titles of Augustus and Princeps. Octavian, the grandnephew and adopted son of Julius Caesar, became the first emperor after defeating Mark Antony at the Battle of Actium in 31 BC and annexing Egypt.
The Western Roman Empire ended in 476 AD, when Romulus Augustulus was forced to abdicate to the Germanic warlord Odoacer. Most chronologies use this date as the conventional endpoint for the western empire.
The Crisis of the Third Century lasted from 235 to 284 AD and was a period of invasions, civil wars, economic disorder, and epidemic disease. By 258, the empire had fragmented into three competing states. The Plague of Cyprian, possibly smallpox, broke out in 251 and caused large-scale mortality during this period.
The Five Good Emperors were Nerva, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius, who ruled in succession from 96 to 180 AD. Each adopted his successor based on merit during his own lifetime, and their period is considered one of the most prosperous in Roman imperial history.
Diocletian created the Tetrarchy in 293 AD to address the empire's ungovernable size, dividing rule among two senior emperors and two junior emperors across four regional quarters. It functioned well while Diocletian and his co-augustus Maximian ruled, but collapsed almost immediately after both abdicated on the 1st of May 305, as competing dynastic claims tore the system apart.
The Eastern Roman Empire ended on the 29th of May 1453, when Ottoman forces breached the walls of Constantinople and the last emperor, Constantine XI, died in the fighting. The eastern empire had outlasted the western empire by nearly a thousand years.