The correct answer to this open question is the following.
Unfortunately, you did not attach the statement made by your teacher. We do not know what he/she said. We cannot explain the meaning of a statement from your teacher about the Roman Empire, because you forgot to include it.
What we can do for you is to write a timeline with the most important events that influenced the fall of the western Roman Empire.
235. The beginning of the Imperial crisis with the rebellion and anarchy of the military. Severus Alexander is assassinated by his troops.
285. Diocletian divides the empire and orders four caesars to help rule the regions of the empire. This period is known as the Tretrarchy. Civil war starts when Diocletian abdicates.
306. Constantine defeats Maxentius and instills Christianity in the Empire. He became the sole emperor of Rome and move the capital to Constantinople, modern-day Turkey.
378 The Visigoths kill emperor Flavius Julius in the Battle of Adrianople .
410. Alaric, King of the Visigoths captured Rome.
440. Atila, leader of the Huns, attack the empire.
476. Odoacer captured Rome and deposed the last emperor Romulus Augustus.