I'm new to DC motors. When we attach electrical loads to a DC motor, the shaft is harder to turn by hand. Is there a way to make it still easy to turn while there's an electrical load?
[Is] there a way to make it still easy to turn while there's electrical load?
No. This is due to a principle known as conservation of energy. You're using the motor backwards as a generator. (This is fine.) The generator is supplying electrical energy to the connected electrical load. This energy has to come from somewhere and it's coming from the mechanical energy used to turn the generator, which you are providing. (In turn, you're getting that energy from chemical energy in the food you eat.)
You should notice that as you connect more electrical load to the generator it becomes harder to turn, as you now need to supply more energy.
This is easy. Simply remove the magnetic field, and the rotor can spin with only bearing friction and windage. Of course, it won't deliver any electrical power to the load.
If it's a wound field motor like a "universal" motor (or a wound field alternator in a vehicle) you just need to disconnect power from the field winding.
If it's a permanent magnet motor...
Or reduce the load.
But if you're trying to make a generator produce electric power without consuming mechanical power, forget it.