Because of the absence of teacher Hugo and the class 3a on Monday there are several open substitutions that have to be dealt with.
The changes in the regularly scheduled lessons are displayed in the timetable.
Open one timetable for classes ("Timetable | Class schedule big") and one for teachers ("Timetable | Te. schedule big").

All changes in the daily updated timetable are displayed in red script. In the timetable of the class 3b you can see that the first two periods have to be covered because of the absence of teacher Hugo. Hugo's absence can be seen in his timetable. The respective day is marked with "Absent".
The changes of all periods involved are displayed as a list in the substitution window (right mouse click | Substitutions) .
Compared to the regular timetable there are two changes in the first period:

First, the release of teacher Rub because of the absence of the class 3a and second, the open substitution of class 3b because of the absence of teacher Hugo. An open substitution is marked with three red question marks ???. This open substitution has to be filled with a suitable subtitute teacher.
Flash-Demo:
Schedule Substitutions