How would I restrict Movement in Air?

I made a Top-down 3D Movement Script where you intentionally can't Jump. But I also don't want, that you can walk in Air. When you're in Air, you should just fall.

This Video shows, how you can move in the Game. And around second 14, it starts showing how you're supposed to be moving. https://youtu.be/Zm2k2PF8BCQ

This is the Script I need help for restricting Movement in Air: using System.Collections; using System.Collections.Generic; using UnityEngine;

 public class Player_Move : MonoBehaviour
 {
     //Movement Variables (Next 3 Code Lines):
     public float walkSpeed;
     private float xInput;
     private float yInput;
     
     //Floor Check Variables (Next 4 Code Lines):
     public Transform floorCheck;
     public float floorCheckRadius;
     public LayerMask floorLayer;
     private bool isOnFloor;
     
     // Start is called before the first frame update
     void Start()
     {
         
     }
 
     // Update is called once per frame
     void Update()
     {
         //Floor Check Code:
         isOnFloor = Physics.OverlapSphere(floorCheck.position, floorCheckRadius, floorLayer).Length > 0;
         
         //Code to get Input for Player Movement (Next 3 if-Statements):
         if (isOnFloor = true) //Only check for Input if Character is on Floor
             {
             if (Input.GetAxisRaw("Horizontal") != 0)
             {
                 xInput = Input.GetAxisRaw("Horizontal");
                 yInput = 0;
             } else
             if (Input.GetAxisRaw("Vertical") != 0)
             {
                 yInput = Input.GetAxisRaw("Vertical");
                 xInput = 0;
             }
         }else //Hinder Input if Character is not on Floor (This else-Statement):
         {
             xInput = 0;
             yInput = 0;
         }
         
         transform.Translate (xInput * walkSpeed, 0, yInput * walkSpeed); //Make the Player Walk according to Input
         
         //Code to restrict diagonal Movement (Next 2 if-Statements):
         if(Input.GetAxisRaw("Horizontal") == 0)
         {
             xInput = 0;
         }
         if (Input.GetAxisRaw("Vertical") == 0)
         {
             yInput = 0;
         }
     }
 }