1. I can't use the default functions (gesture recognition, line following, and obstacle avoidance) after starting mBot Mega, why?

You need to update the firmware to the factory version before you use mBot Mega. For details about firmware update, see Updating the Firmware of mBot Mega.

2. Why does the previous program disappear after I update the firmware?

mBot Mega can store only one program. Updating the firmware also restores the ex-factory program and therefore the previous program is overwritten. 

3. I can control mBot Mega with my mobile phone or Makeblock Bluetooth Controller through Bluetooth but can't upload a program with a Bluetooth dongle, why?

The Megapi main control board doesn't support firmware update, upload programs, or online debugging with a Bluetooth dongle.

4. Why do I need to install the electronic modules in the positions specified in the building instructions when I am to use mBlock 5 to program mBot Mega?

To use the default functions, the sensors need to be connected to the positions specified in the preset programs. Otherwise, the default functions can't be used properly.

5. MegaPi doesn't support multi-threading, why?

Due to resource limit, the single-chip Arduino doesn't support multi-threading currently.