PlayerMovement

 using UnityEngine;

 public class PlayerMovement : MonoBehaviour
 {
     public CharacterController controller;

     public static float NORMAL_SPEED = 12f;
     public static float FAST_SPEED = 20f;

     public float speed = NORMAL_SPEED;
     public float gravity = -9.81f;
     public float jumpHeight = 3f;
     public Transform groundCheck;
     public float groundDistance = 0.4f;
     public LayerMask groundMask;

     public Vector3 velocity;
     bool isGrounded;

     void Update()
     {
         isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);
         if (isGrounded && velocity.y < 0)
         {
             velocity.y = -2f;
         }

         float x = Input.GetAxis("Horizontal");
         float z = Input.GetAxis("Vertical");

         if(Input.GetKeyDown(KeyCode.LeftShift))
         {
             speed = FAST_SPEED;
         }

         if (Input.GetKeyUp(KeyCode.LeftShift))
         {
             speed = NORMAL_SPEED;
         }

         Vector3 move = transform.right * x + transform.forward * z;

         controller.Move(move * speed * Time.deltaTime);

         if(Input.GetButtonDown("Jump") && isGrounded)
         {
             velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
         }

         velocity.y += gravity * Time.deltaTime;

         controller.Move(velocity * Time.deltaTime);
     }
 }