When you get a question like that, it helps to know dates, because then you can line things up in chronological order by date. But even if you don't know dates, often if you know the story of how the history went, the events line up in a natural order according to the flow of the story.
So, by date (and story line)
AD 324 - Constantine began construction of Constantinople. When Constantine moved the capital to the East, that already weakened the status of Rome in the West.
AD 337 - Constantine died (which obviously had to happen after his time as emperor and builder of Constantinople)
AD 455 - Vandals sack Rome. Roughly a century after Constantine moved the headquarters of the empire to the East, the western power center fell to other forces.
AD 527 - Justinian became emperor in Byzantium. Now the only "Roman" empire was in the East, known to us as the Byzantine empire. ("Byzantium" was the name of the city before Constantine had renamed it Constantinople.)
