Player movement

Questions & Answers
1

I have my controls but my character movement won’t move whatever i do this is my update but i cant hold down my movement buttons to move.

void Update () {

    //Your controls.
    if (Input.GetKeyDown(KeyCode.A))
    {
       transform.Translate(Vector2.left * speed); 
    }

    if (Input.GetKeyDown(KeyCode.D))
    {
        transform.Translate(Vector2.right * speed);
    }

    if (Input.GetKeyDown(KeyCode.Space))
    {
        body.AddForce(Vector2.up * jumpSpeed, ForceMode2D.Impulse);
    }
}
2

Instead of using if statement for every key, you should use Input.GetAxix() for moving player left or right. I wrote you some comment so you can see how it works.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerMovement : MonoBehaviour {

    [SerializeField]
    private float maxSpeed = 10f; // Maximum speed player can have

    //Facing of player. It can be Left or Right.
    bool facingRight = true; 

    // Rigidbody 2D component you need to add to Player GameObject
    private Rigidbody2D rigidbody2D;


	void Start ()
    {
        rigidbody2D = GetComponent<Rigidbody2D>();	
	}
	
	// It is better to do this things like movement in Fixed Update
	void FixedUpdate ()
    {
        ///<summary> You dont need if statement to check GetKeyDown(). GetAxix("Horizontal") will check if Player pressed keys (Left or Right Arrow, A or D).
        ///You can also change this in Input Settings if you want 
        ///</summary>
        ///
        float move = Input.GetAxis("Horizontal");

        rigidbody2D.velocity = new Vector2(move * maxSpeed, rigidbody2D.velocity.y);
        
        if(move > 0 && !facingRight)
        {
            Flip();
        }
        else if (move<0 && facingRight)
        {
            Flip();
        }
    }
/// <summary>
/// This function Flip player Sprite
/// </summary>
    void Flip()
    {
        facingRight = !facingRight;
        Vector3 theScale = transform.localScale;
        theScale.x *= -1;
        transform.localScale = theScale;
    }
}