Can I force the generator to place certain lesson on certain position?

You can place the lesson into the timetable before the generation and then lock them. The generator will not move the locked cards. Locked cards are marked with small stripe in the lower right corner:

If you click right mouse button you can specify also the clasroom for this lesson:

IMPORTANT: try to avoid locking lessons unless really necessary. It is better to specify the timeoffs for teacher, classes, subjects etc. If you lock many cards you can make the timetable impossible to generate. Sometimes one card locked on unlucky position can spoil the whole timetable.