Divide Objects Below

This one's a little odd since it breaks a fundamental rule of Illustrator: Make sure you have selected all the objects you want to work with. In the case of the command called Divide Objects Below, it won't work if more than one object is selected. Simply create the object that will do the dividing and position it over the other (filled) object(s) to be divided. Then choose Object>Path>Divide Objects Below. (If you have too many objects selected, it won't work and you'll get a warning dialog.)