[SOLVED] Invalid Controls

I started to use ProjeQtOr, but I have a problem. I have created a task (activities) but now, when I try to change the status to done, I receive this error:
Invalid controls.
the field 'result' is mandatory
How I can resolve it?

Just enter some text in the field "result".
A bit more explanation ?
Each status has flags "set to handled", "set to done", "set to closed". These flags have impact on corresponding checkboxes when selecting new status.
So by default, when you select new status as "done" the "is done" flag is checked.
On activity type, there is an option "result is mandatory on done status". When this option is checked, if you select a new status that will check the " is done" flag, the field result becomes mandatory (you can check this easily as the field gets a red left border).
So you have two options : either enter result field, or change option "result is mandatory on done status" on activity type if you don't need a result for these activities.

Is it right to use activities for subdivide the project in tasks?

Of course
Projects can be split into sub-projects. Sub-project can also be split into sub-sub-project. And so on without limit.
Pojects can also be split into tasks. Tasks can be split into sub-tasks. Sub-tasks can also be split into sub-sub-tasks. And so on without limit.
(I guess you get the trick now)
This is called Work Breakdown Structure (WBS) and is basics of project management (and very important for big projects) : split work into elementary tasks that will be humanly understood (IMO, tasks of 5-10 days is a good scale).

The "result" is usefull to have an axplanation about the task from the person who have done it?

Exactly.