Character won't move forward, only jump.

Just started with unity (and programming). I looked up some videos and tried to learn how to make a character move and jump. Everything works fine, but it seems when I move forward he just jumps. And moving backward does nothing. I checked the inputs in my settings and none of them were changed.

Here's the script:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ToasterMovement : MonoBehaviour 
 {
     public float speed = 6.0F;
     public float jumpSpeed = 8.0F;
     public float gravity = 20.0F;
     private Vector3 moveDirection = Vector3.zero;
     void Update() {
         CharacterController controller = GetComponent<CharacterController>();
         if (controller.isGrounded) {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             if (Input.GetButton("Jump"))
                 moveDirection.y = jumpSpeed;
             
         }
         moveDirection.y -= gravity * Time.deltaTime;
         controller.Move(moveDirection * Time.deltaTime);
     }
 }