I don't understand how electrons can occupy another energy level without filling up entirely the previous energy level's last subshell like for example, Iron's electron configuration is 1s2 2s2 2p6 3s2 3p6 3d6 4s2. Now the d-subshell of the 3rd energy level hasn't been entirely filled out so how can the electrons start filling up the 4th one without completing the 3rd one? I'm confused. Can someone please help? Thank You. 
