Honestly who cares why he wants to know, he is aking either because he wants to open one in flight or out of curiosity. Either way no harm can come from explaining and all this can be found online, not an industry secret.
No you normally can not open a door in flight. The doors in the cabin are plug type door so they have to come into the cabin slightly before being rotated outwards. When the aircraft is pressurized you would not be strong enough to overcome the forces as would have to pull the door in against the pressure. If there is a decompression then yes you could open a door.
When the door is armed the emergency slide should auto inflate if the door is opened from inside the cabin. If it is disarmed it should not inflate when opened from the inside. If opened from the outside this action should also disarm the slide. There is also a handle to manualy inflate the slide if it does not inflate and you need to use it.
Cargo doors are not plug type but have multiple locks holding them in place. I think all incidents where doors have opened in flight of modern airliners have been cargo doors where the locks failed.
You will however get in lots of trouble and arrested if you are seen trying to open a door in flight, which you can't do anyway.