How can i make my player movement script for a box better?

I am a begginer in proggraming so I don't know much about player movement in Unity.Here is my script for moving back and forth,rotating sideways and for jumping:

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent(typeof(Rigidbody))]
 public class PlayerMovement : MonoBehaviour
 {
     public float forwardForce = 30f;
     public float sidewaysForce = 150f;
     public Vector3 jump;
     public float jumpForce = 2.0f;
     public bool isGrounded;
     Rigidbody rb;
     void Start()
     {
         rb = GetComponent<Rigidbody>();
         jump = new Vector3(0.0f, 2.0f, 0.0f);
     }
 
     void OnCollisionStay()
     {
         isGrounded = true;
     }
 
     void FixedUpdate()
     {
         var x = Input.GetAxis("Horizontal") * sidewaysForce * Time.deltaTime;
         var z = Input.GetAxis("Vertical") * forwardForce * Time.deltaTime;
 
         transform.Rotate(0, x, 0);
         transform.Translate(0, 0, z);
         if (Input.GetKeyDown(KeyCode.Space) && isGrounded)
         {
 
             rb.AddForce(jump * jumpForce, ForceMode.Impulse);
             isGrounded = false;
         }
     }
 }

The problem is that whenever i try to jump somewhere and then fall,there is a possibility that my box rotates its axis soo whenever i press W (which is represented by x axis) and the box has rotated so that its x axis is oriented to -y axis,the ball doesnt move forth ,it moves in the ground. Can someone please rewrite my script soo that my movement won't depend on x,y,z axis anymore. Thanks!