unable user from click certain item in drop down list if certain fields is unfilled

[maria_mj]

I have to make a control for my gridview.
in edit mode, user can select phase from phase drop down list in phase column.
The phase drop down list contain 3 item : definition phase, confirmation phase and delivery phase.
Before the phase column, there have a few column. I want the user to fill that field before they can select "delivery phase" in phase drop down list.
It is because, during delivery phase, the value in that field is compulsory. So, user cannot choose that phase if they did not fill that field.
I dont know how to do that...
please help me...