Having just one engine operating means you won't have the maximum thrust power for take off, but you'd be able to fly and land just fine. But while a plane can fly on one engine, it is very rare for an engine to go down in the middle of a flight.

Yes, all modern commercial multi-engine aircraft are specifically designed, tested, and certified to fly safely—and even climb—on a single engine. This capability is a requirement for "ETOPS" (Extended-range Twin-engine Operational Performance Standards) certification. If an engine fails during the most critical phase, such as takeoff, the remaining engine has enough thrust to allow the pilots to safely gain altitude, circle back, and land. Once at cruising altitude, if one engine fails, the plane cannot maintain its high altitude and will perform what is known as a "drift down" to a lower, denser altitude (usually around 15,000 to 25,000 feet) where the remaining engine can provide sufficient lift. Pilots train extensively in flight simulators to handle "engine-out" scenarios, focusing on using the rudder to counteract the asymmetrical thrust. While it is an emergency situation that requires an immediate landing at the nearest suitable airport, it is not a "death sentence" for the aircraft.