Newbie query :simple program, one form - why multiple units?

From: RedPenn (basenjinut_at_hotmail.com)
Date: 03/30/05 

Date: 29 Mar 2005 16:56:44 -0800

The lecturer has insisted that we write our code in multiple units,
even tho' the application does very little. I have a working version,
but did not pass the assessment as I had all the code in one unit -
the form unit. He expects us to have a "black-box" or "logic" unit, as
well as other units for common routines (after only three projects,
how many of these common routines will we have already?).
Picture the form - an edit box and two combo boxes. Three buttons.
User can enter an ID, and select a course and a result and on button
click a record is added to a dynamic array. User can enter/select the
same data and delete the record if it exists.
Student enhancement to the requirement is to allow the user to see in
a memo/list box which records have been added, and to select one in
the memo/list box for deletion.
As soon as i try to put the hidden (non-interface-related) procedures
into this logic unit, my app falls to custard. (I know about adding
the second unit's name to the "uses" list in the interface of the
form's unit.)
Query (or "b**chy vent, if you like)- if my one unit app works, how do
i get zero marks?
Multiple units - i can see the point of mastering their use for major,
complex apps, and I'd love to be able to say "Ah, now i get how to do
it!" But this tiddly little app?

Your thoughts welcome! (Not answers to the how, but opinions on the
"WHAT?"