Key combinaison

when I keep pressed two key on the keyboard and I want to
press another in the same time, some key dont works

exemple : I wnat to keep pressed these keys for some
raison : [ an ]
But i am not abble to write or use letter e f g m s r and
other key like F1!!

How can I make the computer recognize the third key? Do I
need a special keyboard ????? =)