Short answers, pulled from the story.
The Roman Empire began in 27 BC when the Roman Senate granted Octavian overarching military power and the title of Augustus. Octavian, the adopted son of Julius Caesar, became the first Roman emperor through a carefully constructed political theater that preserved the illusion of the Republic while concentrating absolute authority in one man.
Most chronologies place the end of the Western Roman Empire in 476 when Romulus Augustulus was forced to abdicate to the Germanic warlord Odoacer. Odoacer ended the Western Empire by declaring Zeno sole emperor and placing himself as Zeno's nominal subordinate while ruling Italy alone.
The Eastern Roman Empire ended in 1453 when Constantine XI Palaiologos died in battle against Mehmed II and his Ottoman forces during the siege of Constantinople. Mehmed II adopted the title of caesar in an attempt to claim a connection to the former Empire, a claim that was soon recognized by the Patriarchate of Constantinople.
Latin and Greek were the main languages of the Empire, but the Empire was deliberately multilingual with the main desire of the Roman government being to make itself understood. The wide use of Koine Greek enabled the spread of Christianity and reflects its role as the lingua franca of the Mediterranean during the time of the Empire.
After Germanic tribes wiped out three legions in the Battle of the Teutoburg Forest in 9 AD, the number of legions was increased from 25 to around 30. The exact size of the Imperial legion has been estimated to range from 4,800 to 5,280 soldiers.
The Empire underwent a 49-year crisis in the 3rd century that threatened its existence due to civil war, plagues, and barbarian invasions. The Gallic and Palmyrene empires broke away from the state, and a series of short-lived emperors led the Empire before it was later reunified under Aurelian.