Control Air movement with character controller

Hi guys, I am now working on my 3D 3rd person running game now but I have got a big problem on my character controller.

Problem: I want the player can control the character move to the left or right while the player controller is not Onground. I tried to remove if (controller.isGrounded ) statement in the MovementControl() but when I removed it, the character control become not effected by the gravity and cannot jump too. I know the problem is in the MovementControl() but I have already spent one day to fix it but I can't. Can some one please help?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class OceanWorldBMControl : MonoBehaviour {
 
     public Animator anim;
     public CharacterController controller;
     public GameObject water;
 
     public float speed = 6;
     public float sprintSpeed;
     public float currentSpeed; 
     public float jumpSpeed = 8.0f;
     public float gravity = 20.0f;
     public float turnspeed = 10.0f;
     public bool OnGround = true;
     public bool DoubleJumped;
     public bool isSprinting = false;
     public bool isBootsed = false;
     public bool isSwimming = false;
     public bool isPressSprint;
 
 
     public Slider staminaBar;
     public int maxStamina;
     private int staminaFallrate;//how fast it fall
     public int staminaFallMult;
     private int staminaRegainrate;// how fast it recover
     public int staminaReganinMult;
 
     private double boostTime = 20f;
     private float boostedSpeed;
     public float boostTimeFallrate = 1f;
 
     private Vector3 moveDirection = Vector3.zero;
 
 
     // Use this for initialization
     void Start () {
 
         currentSpeed = speed;
         sprintSpeed = speed * 2;
         staminaBar.maxValue = maxStamina;
         staminaBar.value = maxStamina;
 
         staminaFallrate = 1;
         staminaRegainrate = 1;
     }
 
     // Update is called once per frame
     void Update () {
         AnimationControl();
         AirTime ();
         MovementControl ();
         Sprinting ();
     }
 
     void MovementControl()
     {
         /*float rotation = Input.GetAxis ("Horizontal") * turnspeed;
         rotation *= Time.deltaTime;
         transform.Rotate (0, -rotation, 0);*/
 
         if (controller.isGrounded ) {
             moveDirection = new Vector3 (Input.GetAxis ("Horizontal")*speed, 0, Input.GetAxis ("Vertical") * currentSpeed);
             moveDirection = transform.TransformDirection (moveDirection);
         }
 
         if (OnGround && Input.GetButtonDown ("Jump")) {
             moveDirection.y = jumpSpeed;
         }
 
         if (!OnGround && Input.GetButtonDown ("Jump") && !DoubleJumped) {
             moveDirection.y = jumpSpeed;
             DoubleJumped = true;
         }
 
         moveDirection.y -= gravity*Time.deltaTime;        
         controller.Move (moveDirection * Time.deltaTime);
     }
 
     void AnimationControl()
     {
         if (OnGround && Input.GetButtonDown ("Jump")) {
             anim.SetTrigger ("isJumping");
         } 
 
         if (!OnGround && Input.GetButtonDown ("Jump") && !DoubleJumped) {
             anim.SetTrigger ("DoubleJump");
 
         }
         if (controller.isGrounded) {
             anim.SetBool ("isAir", false);        
         } 
         else {
             anim.SetBool ("isAir", true);
         }
         if (Input.GetKey (KeyCode.S)) {
             anim.SetBool ("isRunning", true);    
         } 
         else {
             anim.SetBool ("isRunning", false);
         }
         if (staminaBar.value > 0 && isSprinting || isBootsed) {
             anim.SetBool ("isSprinting", true);
         } 
         else {
             anim.SetBool ("isSprinting", false);
         }
         if (Input.GetKey (KeyCode.W)) {
             anim.SetBool ("isBack", true);    
         } 
         else {
             anim.SetBool ("isBack", false);
         }
         if (isSwimming) {
             anim.SetBool ("isSwimming", true);
         } else {
             anim.SetBool ("isSwimming", false);
         }
     }
 
     void AirTime()
     {
         if (!controller.isGrounded) {
             OnGround = false;
         } else {
             OnGround = true;
             DoubleJumped = false;
         }
     }
 
     void Sprinting(){
 
         if (Input.GetKey (KeyCode.P)) {
             isPressSprint = true;
         } else {
             isPressSprint = false;
         }
 
         if (Input.GetKey (KeyCode.S) && Input.GetKey(KeyCode.P) && !isSwimming) {
             isSprinting = true;
         } 
         else {
             isSprinting = false;
         }
 
         if (staminaBar.value >= maxStamina) {
             staminaBar.value = maxStamina;
 
         } else if (staminaBar.value <= 0) {
             isSprinting = false;
             staminaBar.value = 0;
             currentSpeed = speed;
         }
 
         if (!isSprinting && !isBootsed) {
             currentSpeed = speed;
             staminaBar.value += Time.deltaTime / staminaRegainrate * staminaReganinMult;
         }
 
         if (isSprinting && !isBootsed && isPressSprint) {
             currentSpeed = sprintSpeed;
             staminaBar.value -= Time.deltaTime / staminaFallrate * staminaFallMult;
         }
 
         if (isSprinting && isBootsed ) {
             currentSpeed = sprintSpeed;
             staminaBar.value -= Time.deltaTime / staminaFallrate * staminaFallMult;
             boostTime -= Time.deltaTime * boostTimeFallrate;
             if (boostTime <= 0) {
                 isBootsed = false;
                 currentSpeed = speed;
                 boostTime = 20f;
             }
         }
         if (!isSprinting && isBootsed) {
             currentSpeed = sprintSpeed;
             staminaBar.value += Time.deltaTime / staminaRegainrate * staminaReganinMult;
             boostTime -= Time.deltaTime * boostTimeFallrate;
             if (boostTime <= 0) {
                 isBootsed = false;
                 currentSpeed = speed;
                 boostTime = 20f;
             }
         }
 
         if (isSprinting && !isPressSprint) {
             staminaBar.value += Time.deltaTime / staminaRegainrate * staminaReganinMult;
         }
     }
 
     void OnTriggerEnter(Collider other){
         if (other.gameObject.tag == "water") {
             isSwimming = true;
             Debug.Log ("inWater");
         } 
     } 
     void OnTriggerExit(){
         isSwimming = false;
     }
 }