WD2007 and other versions never did this the way we expected.
http://www.terryscomputertips.com/computers/form-underlining-in-microsoft-word.php explains this and supplies workarounds.
The computer I am using has Windows 2003.
I received a Word document (saved as a Word 97-2003) that I need to turn into a Form. I have saved it as a Word template (I also saved it as a Word 97-2003 Template to see if that fixed my problem and it didn't). I turned on the design mode in the developer tab and then added Text Form Fields (using the Legacy tools). Certain fields need to be underlined so I highlighted those fields and used ctrl U (and also tried the underline button on the Home tab). In the form the underline shows up. When I protect the form, the underline still shows up. But as soon as I type anything into any of those underlined fields, the underlining disappears.
How do I get the underlining to stick? Microsoft Help tells me to do exactly what I did and I haven't had any success finding answers online. There has to be one little thing I'm doing wrong. Any suggestions would be greatly appreciated.