Flying Orb Strafe Help C Sharp

In this little game I am working on, I have a little flying orb. I am using the mouse look script from the first person controller, but the movement script is just something I am trying to make to teach myself. Right now it works well. I can move forward with the W key and turn with mouse look and I can fly around using that, but I want to be able to strafe and go backwards as well. Here is the script.
using UnityEngine;
using System.Collections;

public class Move : MonoBehaviour {
	
	public float forwardSpeed;
	public float sidewaysSpeed;
	
	// Use this for initialization
	void Start () {
	
	}
	
	// Update is called once per frame
	void Update () {
	   
		if(Input.GetKey(KeyCode.W)) {
        transform.Translate(Vector3.forward * forwardSpeed * Time.deltaTime);
        transform.Translate(Vector3.up * Time.deltaTime, Space.World);
		transform.Translate(Vector3.down * Time.deltaTime, Space.World);	

			
		
			if(Input.GetKey(KeyCode.Space)) {
				forwardSpeed = 50f;
				
			}	
		
			else {
				forwardSpeed = 20;
			}
		}
	}
}

Could someone figure out how to make him strafe and move backwards? Also is the way I have the speed boost set up the most efficient. It seems like checking every frame to if the space bar isn’t down to set speed to 20 like a bit much. Any help would be much appreciated.

Why are you translating upward and downward, from the same amount and at the same moment ? They cancel each other.

Anyway, to move backward you need to do like the forward movement → input detection, Translate( Vector3.back * backSpeed * Time.deltaTime );. For the straff, the best is probably an animation.