There are many suggestions from FET users to improve FET's UI. It will be nearly impossible to find a UI that meets everybody's needs, I know. But let me just present, what was confusing me for a while. Maybe I'm not the only one.
Dialogue "Modify Activity":
Changing the state of the check box "Active" does not change the active/passive state of all subactivities that belong to the same activity. It affects only the single subactivity that shows in the tab (= the one that has been highlighted bevor the dialog was opened). If I want to activate the whole activity I have to click through all of the tabs. (It took some time until I understood this.)
Since there are two dialogs, one for activities and one for subactivities the (novice) user does not expect the possibility of editing individual subactivity properties in the activities dialog (because individual activation and individual duration of subactivities can be chosen in the subactivities dialog).
By the way, is it really necessary to have two dialogs? I could imagine a check box "subactivity only" in a common dialog for both, activities and subactivities. This would make this comment obsolete.
It is done like that, because there you have also the duration for each subactivity. I'll add this suggestion in the TODO, because I am not sure I like it. Modify subactivities means to modify the teacher/subject/students/activity tags/number of students for each subactivity, not the duration or active state.