Q: Car dies, goes into safe mode after I jump the battery.

While you may be jumping your battery correctly, your Audi A3 is equipped with an immobilizer that keeps the vehicle from being started without use of the proper key programmed for the vehicle. What you are describing sounds like a common issue where the computer in the vehicle is not able to properly communicate with the key to start the vehicle. I would try starting the vehicle with your spare key as your first step. If the vehicle still will not start, and if you feel comfortable disconnecting the battery, try detaching the positive and negative battery wire connectors for at least 30 minutes, then reconnecting the battery and attempting to start your A3 once again. If your vehicle still does not start, I would then think it is best to get in contact with a a certified technician for a starting system inspection. A mechanic will have the knowledge and tools needed to communicate with the vehicle’s computer and determine why the vehicle is stuck in safe mode and help get you back on the road again.