currentSpeed doesnt change when pressing Shift (C#)

Hello. I'm fairly new to programming and game making so I'm setting up my first character controller.

What I have is a Player who can walk along the x and z axis. I have 3 floats in my script named "walkSpeed", "sprintSpeed" and "currentSpeed" and I would set them up in the Inspector. ie: walkSpeed = 5 and sprintSpeed = 10.

I want to use the walkSpeed as the normal Speed and the sprintSpeed only if the left Shift key is pressed.

What I got for now:

     public float walkSpeed;
     public float sprintSpeed;
     public float currentSpeed;
 
     void Update () {
 
         if (Input.GetKeyDown("left shift"))
             currentSpeed = sprintSpeed;
         else
             currentSpeed = walkSpeed;
 
         float verMov = Input.GetAxis("Vertical") * currentSpeed;
         float horMov = Input.GetAxis("Horizontal") * currentSpeed;
 
         verMov *= Time.deltaTime;
         horMov *= Time.deltaTime;
 
         transform.Translate(horMov, 0, verMov);
 
     }

In my understanding the currentSpeed should change to 10 IF the left shift key is pressed and ELSE the currentSpeed should stay 5. but when i start the game the currentSpeed stays 5 even when im pressing left shift.

I would love to hear your tips regarding this code. And best you can do is finding a way to let this script work and not posting your script and saying "replace this with yours it works 100%".

Thank you :)

EDIT: when I revert currentSpeed = sprintSpeed and currentSpeed = walkSpeedto sprintSpeed = currentSpeed and walkSpeed = currentSpeed the walkSpeed goes instantly to 0 and if I press the shift key the sprintSpeed changes from 10 to 0.

That shows that the code recognizes that the key is pressed.