If I've got a PS/2 connect to my spectrum then certain keys or buttons - like the NMI button - are mapped to either F-keys or key combinations.
This isn't a complete list, but useful (to me).
|Edit||Shift + 1|
|Caps lock||Shift + 2|
|Backspace||Shift + 0|
Keys that are required when in some other mode that I don't know the name for (i.e. running
|Quote "||Ctrl + p|
|Plus +||Ctrl + k|
|Comma ,||Ctrl + n|
|Period .||Ctrl + m|
|Colon :||Ctrl + z|
There's also this document that shows PS2 mappings to the spectrum, useful when you're forced to use the "48K style keys".