programming style

I have a basic understanding of the ladder logic programming structure. I understand that its possible to program subsets that enclose certain rungs. I’m confused as to how a “if-then” structure of programming would work. As an example: if x0 = 170 degrees F then turn on y0, y02, y22 (This would be a subset, one input to control various outputs). That subset would run until the next input became true (i.e. x1 = 180 degrees F and then turn on some other outputs.) This subset would run until a timer kicked the program to the next subset. Is this the best way to program the PLC? Am I correct in assuming that once the program has ran a subset it won’t go back and try to run it again?

PLCS programs run in a continuous loop.
Generally when you call a subroutine, the subroutine runs, and then returns back to the next rung in the main program.

Great drawing. We have tried to explain subroutines before and that is the best approach yet.

If I wanted the program to just run once is there a way I can trick the PLC into doing one loop? I would want the PLC to complete each subroutine in order and then when it starts the loop again the program would not run or search for inputs that aren't true. Perhaps nestling the an entire program including the subroutines and have two outputs; one being the first program w/ subroutines and the second being empty. Once the program is run a counter would trip the routing to the second program, which would be empty. end program.