Show/hide fields based on other fields or events is a pretty standard one around here. Depending on which form option (Adobe or FPM or other) you use, it will dictate which way to do it.
For Adobe forms, you can use client side (in form) Javascript/Formcalc to control the "presense" of the field(s) based on events in other fields (like drop down selection) along with using user events too if you like.
For FPM forms, you can set up "user events" in config and then "bind" those to an action on your form (like selecting an entry in a dropdown), then you use a generic service to "catch" the event and set the UI attributes of your fields to show/hide as needed. I just did a couple of blogs about this:
HCM Processes & Forms: Making "User Events" Useful Again!
HCM Processes & Forms: Taking control of your FPM form tables...by brute force! (haha)
HCM Processes & Forms: Smart FPM F4! (or “how to step up your drop-down list game”)
...and others have blogged on it too (also search for "ui attributes).
It is pretty easy once you get the hang of what is going on and where. Good luck!