formatting text box depending on input from another text box

Hi

im creating a form in which i need to limit the input into a text box based on the value of another text box. for example if text box "wpds" has UB36 input into it, I then want text box "pass"  to only allow the input of "Root","Hot", "Fill" or "Cap".  But if text box "wpds" has UF49 input into it, i want text box "pass" to only allow the input of "Layer1" "Layer2" or "Layer3".  Any input into this will be greatly appreciated. thanks