Apparently Java is the deafult case. I have no problem when working with a "foo.java" file to insert a "line comment". Then "//" appears correctly in front of the selected line.
However, when I try to do same for a "foo.f90" file, jEdit ignores the command (instead of inserting "!").

My setup for the editing dialog: (with WindowsXP)
Utilities/Global Options/jEdit/Editing/Change Settings for mode: fortran
File name glob: *.f90
Default edit mode: fortran
Then push buttons: Apply & OK

Then from the "Edit" drop-down menu:
Edit/Source/Line Comment

Result on the highlighted line: command IGNORED
Although, as I said, with a Java file I had no problem doing the same.

Why?

First-time user.