Short answers, pulled from the story.
The Roman Republic began traditionally in 509 BC with the overthrow of the monarchy and ended in 27 BC when the Senate granted Octavian the title Augustus, marking the start of the Roman Empire. The Republic therefore lasted roughly 482 years.
The Conflict of the Orders was a struggle between the patrician aristocracy and the plebeian commoners over political rights. It ended around 287 BC when the dictator Quintus Hortensius passed a law making plebeian council decisions binding on all citizens. The conflict opened the consulship to plebeians in 366 BC and eventually broke the patrician monopoly on priesthoods.
Hannibal crossed the Alps in May 218 BC with roughly 100,000 soldiers and 37 elephants, losing nearly half his force to the crossing before defeating Rome at the Trebia river, Lake Trasimene, and most devastatingly at Cannae in 216 BC, where approximately 20 percent of Rome's military-aged male population was killed in a single battle. Eighty senators died at Cannae and many of Rome's Italian allies defected.
The historian Erich S. Gruen argued that civil war caused the fall of the republic rather than the reverse. Contributing factors included the loss of elite cohesion from around 133 BC, wealth inequality, a Senate that protected short-term interests over long-term stability, and the shift of professional army loyalty from the state to individual commanders such as Marius, Sulla, Pompey, and Caesar.
Julius Caesar was a Roman general and statesman who crossed the Rubicon river on the 10th of January 49 BC to seize power, eventually holding the dictatorship, permanent tribunician powers, and the authority to appoint all magistrates. He was assassinated at a Senate meeting on the 15th of March 44 BC by a group of senators led by Gaius Cassius and Marcus Brutus.
The Roman Republic operated through a combination of annually elected magistrates, a permanent Senate, and popular assemblies. Two consuls held supreme civil and military power for one-year terms, each able to veto the other. The Senate controlled finances, provincial assignments, and foreign policy. The plebeian council could pass laws binding on all citizens, and tribunes held veto power over magistrates' actions.