Actually, I finally got it to work. The expressions are fully case sensitive. All field names must be entirely lowercase, and the if statement must use the choice name, not the label. Here's an example of what kind of if statement you would put in the required field on the survey tab:
if( ${doyoulikegis}='no','yes','')
In the above example, if you answer that you don't like GIS, you'd be required to answer the question to which that form expression applies (such as "Why don't you like GIS?")