Some recurrent problems, which I'm tired of having to solve over and over.
In some installations, Ctrl+^ does not produce the superscript, Ctrl+/ does not produce the fraction, etc. This happens to be due ot foreign keyboards (French in my case). Keyboard shortcuts are defined in the file named KeyEventTranslations.tr. This should be located in the directory specified by:
FileNameJoin[{$InstallationDirectory, "SystemFiles", "FrontEnd", "TextResources",
$OperatingSystem}]
However, this is also overridden by MenuSetup.tr which is TextResources/X/ in my case. There I changed the line:
MenuItem["&Superscript", "Superscript", MenuKey["6", Modifiers->{"Control"}]],
for
MenuItem["&Superscript", "Superscript", MenuKey["^", Modifiers->{"Control"}]],
as well as
MenuItem["&Radical", "Radical", MenuKey["2", Modifiers->{"Control"}]],
for
MenuItem["&Radical", "Radical", MenuKey["@", Modifiers->{"Control"}]],
which fixed the problem. Indeed superscript can also be obtained by Ctrl+6, which always work, even after the above substitution.