Three ways to draw a square

One way is to work out the co-ordinates of the four corners, move the pen to an initial position at one of them, then draw a line to each of the others in turn. 
      #include "library.h"
      
      void main()
      { make_window(500, 500);
        move_to(100, 100);
        draw_to(400, 100);
        draw_to(400, 400);
        draw_to(100, 400);
        draw_to(100, 100); }

An alternative method would be to work out where one corner is, and decide on the length of each side. Move the pen to the one known corner, then draw four relatively positioned lines: first increasing x, then increasing y, then decreasing x, and finally decreasing y. 

      #include "library.h"
      
      void main()
      { make_window(500, 500);
        move_to(100, 100);
        draw_relative(300, 0);
        draw_relative(0, 300);
        draw_relative(-300, 0);
        draw_relative(0, -300); }

That is a better method because it can draw a square from any starting point without substantial change. We could make the four line-drawing instructions into a new function, and use that three times to draw three squares. 

      #include "library.h"
      
      void draw_a_square()
      { draw_relative(300, 0);
        draw_relative(0, 300);
        draw_relative(-300, 0);
        draw_relative(0, -300); }
      
      void main()
      { make_window(700, 700);
        move_to(100, 100);
        draw_a_square();
        move_relative(20, 40);
        draw_a_square();
        move_relative(20, 40);
        draw_a_square(); }

The sequence of instructions that draws three squares slightly offset from each other could itself be made into a separate function, easily allowing us to make a pattern of nine squares. 

      #include "library.h"
      
      void draw_a_square()
      { draw_relative(300, 0);
        draw_relative(0, 300);
        draw_relative(-300, 0);
        draw_relative(0, -300); }
      
      void draw_three_squares()
      { draw_a_square();
        move_relative(20, 40);
        draw_a_square();
        move_relative(20, 40);
        draw_a_square(); }
      
      void main()
      { make_window(700, 700);
        move_to(100, 100);
        draw_three_squares();
        move_to(200, 100);
        draw_three_squares();
        move_to(300, 100);
        draw_three_squares();
        move_to(400, 100); }

A third way of drawing a square would be to draw a line of the desired length, then turn by 90 degrees, and repeat four times. 

      #include "library.h"
      
      void main()
      { make_window(700, 700);
        move_to(100, 350);
        set_heading_degrees(45);
        draw_distance(300);
        turn_right_by_degrees(90);
        draw_distance(300);
        turn_right_by_degrees(90);
        draw_distance(300);
        turn_right_by_degrees(90);
        draw_distance(300); }

This is even more flexible, because it could draw a square at any position and at any orientation. We can make the basic square-drawing part into its own function, and use it to draw all sorts of squares at all sorts of positions. 

      #include "library.h"
      
      void draw_a_square()
      { draw_distance(300);
        turn_right_by_degrees(90);
        draw_distance(300);
        turn_right_by_degrees(90);
        draw_distance(300);
        turn_right_by_degrees(90);
        draw_distance(300); }
      
      void main()
      { make_window(700, 700);
        move_to(100, 350);
        set_heading_degrees(45);
        draw_a_square();
        move_relative(200, 0);
        set_heading_degrees(10);
        draw_a_square();
        move_relative(100, -200);
        set_heading_degrees(130);
        draw_a_square(); }