All right, there are a ton of reasons Rome fell, but here are a few:
-Lack of laws restricting inflation: inflation is when money loses its value, so basically you have to spend more to get less and less. For example, if every store in the world decided to start charging $100 for an apple, money would lose its value because $100 would only buy you one piece of fruit. Inflation went rampant towards the end of the Roman Empire, so the market was pretty much dead
-Hired mercenaries: Emperors got lazy and decided to start hiring mercenaries (paid soldiers) to fight in the legion. However, mercenaries lacked the loyalty and discipline that a Roman citizen had, and were often paid even more by enemies of Rome to fight on their side instead. Also, once they were done fighting for the Romans, they might go to another empire's army and teach them all of the tricks that they learned from fighting with the Romans.
-Weak emperors: the last emperors of Rome were inexperienced and often corrupt and self-serving. That's never good for an empire.
Hope that helps!
