input form operation : list

Hi,

I'm trying to add field in the input form operation. two field is dropDown to choise one value.

the first field is the "type operation" and the second the "sub type operation".

My needed is when someone choose the "type operation" the list of "sub-typeOperation" update and show just the value of sub-typeOperation link with "typeOperation".

form example :

 here the list :

When someone choose comercial, i want to see just the subtypeoperation commercial.

I need help

Thanks