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).

Function Key
Edit Shift + 1
Caps lock Shift + 2
Backspace Shift + 0
Break Space

Keys that are required when in some other mode that I don't know the name for (i.e. running .uart):

Function Key
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".