voltage, current, torque and speed in DC motors
I have noticed that sometimes, the motor seems to have little to no
  torque, yet when I limit the current to the motor, it seems to have a
  different behavior.


In a standard (brushed, permanent magnet) DC motor, torque is directly proportional to current. So if you find that the motor sometimes has less torque it's simply because it is drawing less current.


  I want an answer that explains (preferably with formulas) what
  determines voltage, current, torque and speed, in simple terms.


The other question has an answer with formulas, but perhaps the explanation wasn't simple enough for you? 

In simplest terms:-

Voltage is set by the supply. When spinning the motor generates an internal voltage called back-emf, which is proportional to rpm. The difference between the back-emf voltage and the supply voltage is dropped across the motor's internal resistance (Rm), resulting in a current draw according to Ohm's Law (I = V/R). The current produces torque which causes the armature to spin faster, increasing back-emf and reducing current until the torque produced is balanced by internal and external loading. 

If there were no internal losses an unloaded motor would spin fast enough to produce a back-emf equal to the supply voltage. However some current is required to overcome internal mechanical losses (brush and bearing friction, wind resistance) and magnetic losses (eddy currents, hysteresis) which are roughly proportional to speed. The motor's no-load current accounts for all of these internal losses. No-load current does not contribute to output torque, and slightly reduces no-load speed due to voltage drop across Rm.      

When an external load is applied current increases to produce the torque required to match it, and speed reduces due to the increased voltage drop across Rm. As load is further increased the motor slows down more, eventually reaching stall (zero rpm) where there is no back-emf and all the supply voltage is dropped across Rm, resulting in maximum current and torque. 

As loading increases torque increases linearly but rpm decreases linearly, so maximum output power occurs at 50% rpm and torque. Motor efficiency is the ratio of output power (torque x rpm) to input power (Volts x Amps). 

Plotting all these interactions on a graph gives you this:-