Use Toggle Breakpoint (or press F9) to turn breakpoints
on and off. Breakpoints are markers placed on statements
in your program. If you run your program and it encounters
a breakpoint, the program stops on that statement. QBasic
highlights the breakpoint line.
 
To turn a breakpoint on and off:
  1. Move the cursor to the line you want the breakpoint
     on, or to a highlighted breakpoint line.
  2. Choose Toggle Breakpoint (or press F9).
 
Use a breakpoint to pause your program at a point where
you suspect problems, then:
  - Print the values of variables in the Immediate window.
  - Single step your program from the breakpoint forward.