Here's a rundown...
Say your Regular Rotom has 4 attacks.
Ominous Wind, Substitute, Charge, and Discharge.
Now, you go to the room to change it's Form. You click the Lawnmower, and when it changes, it offers you the ability to learn Leaf Storm (in the same way a level up move does- "Rotom want's to learn Leaf Storm. However, Rotom already knows 4 moves. Delete a move for Leaf Storm?").
If you say no, than... That's that. No.
If you were to than change Rotom's form to Heat Rotom, it would than offer to learn Overheat. Again, if you declined, when you changed its form again, it would try to learn that form's special move.
Now, let's say you have a Frost Rotom with Blizzard, Discharge, and Ominous Wind. If you change it's form to Spin Rotom, than, Blizzard will automatically replace itself with Air Slash. You now have a Spin Rotom with Air Slash, Discharge, and Ominous Wind.
Make sense so far?
Now than, say you have a Wash Rotom with Discharge, Ominous Wind, Substitute, and Hydro Pump.
If you revert Wash Rotom back to it's original form, it will have Discharge, Ominous Wind, Substitute, and a blank space. The special move is removed and Rotom gains nothing in return. It's left with an empty slot. However, say you had a Heat Rotom with just Overheat. If you put it back to normal, Rotom would have no moves! That wouldn't work. So in that case, it automatically learns Thundershock, one of Rotom's level 1 moves.
And that's Rotom. Any questions?