Problem Statement

Show a loop which prints random numbers (each number newly generated each loop) from 0 to 19 (inclusive). If a number is 10, stop the loop after printing it, and do not generate any further numbers.
Otherwise, generate and print a second random number before restarting the loop.
If the number 10 is never generated as the first number in a loop, loop forever.

Let's start with the solution:

Loop
{
  Random, var, 0, 19
  output = %output%`n%var%
  If (var = 10)
    Break
  Random, var, 0, 19
  output = %output%`n%var%
}
MsgBox % output