1. // array, that stores if ellipses should be drawn or not
   boolean[] ellipseToggle = new boolean[5];
   
   void setup() {
     size(240, 240);
     smooth();
     // no loop of draw needed, because nothing is animated
     noLoop();
   }
   
   void draw() {
     // clear background
     background(#E0960B);
   
     // draw ellipses if they are "on"
     if (ellipseToggle[0])
     {  
       ellipse(100, 100, 100, 100);
     }
     if (ellipseToggle[1])
     { 
       ellipse(60, 60, 60, 60);
     }
     if (ellipseToggle[2])
     {
       ellipse(20, 20, 20, 20);
     }
     if (ellipseToggle[3])
     {
       ellipse(40, 40, 40, 40);
     }
     if (ellipseToggle[4]) {
       ellipse(10, 10, 10, 10);
     }
   }
   
   void keyPressed() {
     // turn ellipses on/off
     if ((key == 'q') || (key =='Q')) {
       ellipseToggle[0] = !ellipseToggle[0];
     }
     if ((key == 't') || (key == 'T')) {
       ellipseToggle[1] = !ellipseToggle[1];
     }
     if ((key == 'w') || (key == 'W')) {
       ellipseToggle[2] = !ellipseToggle[2];
     }
     if ((key == 'e') || (key == 'E')) {
       ellipseToggle[3] = !ellipseToggle[3];
     }
     if ((key == 'r') || (key == 'R')) {
       ellipseToggle[4] = !ellipseToggle[4];
     }
     // turn all ellipses off
     if (key == ' ') {
       for (int i=0; i< ellipseToggle.length; i++) {
         ellipseToggle[i] = false;
       }
     }
    
     // execute the code inside draw() again
     redraw();
   }

The "if (keyPressed == true)" in your keyPressed() function was not necessary, it is always true when the function is invoked. You could improve the code by adding more varibales, but i thought for understanding i should not change too much from your original code.