How to let If-loop running all the time

i would like to know how i let my program to check the if-statement all the time while the others functions still running.

def back():
   print("back")
counter = 0
while counter <= 1:
   if statement == True:
      print("hey")
      counter += 1
back()

Here the statement has to be True before the function back can be executed, but I want it all at the same time to run