There are three possibilities for moving periods in the timetable:
1.Moving to an empty period
3.Scheduling periods with clashes
Moving to an empty period
If you drag a period away from its position with the mouse, potential so far empty locations suitable for a move will be marked green.
If you drag a period away from its position with the mouse, potential swap partners will be marked green.
Scheduling periods with collisions
You can also generate clashes. Drag a period, drop it on another and select the type of clash you wish to allow.