You'll find the answer to the question you posted in #5 in the description of how the TOF instruction works.
As far as your original question is concerned, if the way you posted it is the way the teacher stated it to you, then it's time to fire your teacher. Somehow, I don't think that's the case. It's a lot more likely that you weren't paying enough attention when the teacher gave the assignment.
In broad terms, the function of a PLC program is to produce an action or maybe a sequence of actions in response to an event. You haven't defined what action(s) your "four-line program with two control relays that works" is supposed to produce. Furthermore, you haven't bothered to define the event that's supposed to intitiate the action.