Subscripts, Bars, and Other Modifiers
Here is a typical palette of modifiers.
Mathematica allows you to use any expression as a subscript.
| Out[1]= |  |
Unless you specifically tell it otherwise,
Mathematica will interpret a superscript as a power.
| Out[2]= |  |
| Ctrl+_ or Ctrl+- | go to the position for a subscript |
| Ctrl++ or Ctrl+4 | go to the position underneath |
| Ctrl+^ or Ctrl+6 | go to the position for a superscript |
| Ctrl+& or Ctrl+7 | go to the position on top |
| Ctrl+Space | return from a special position |
Special input forms based on control characters. The second forms given should work on any keyboard.
This enters a subscript using control keys.
| Out[3]= |  |
Just as Ctrl+^ and Ctrl+_ go to superscript and subscript positions, so also Ctrl+& and Ctrl+4 can be used to go to positions directly above and below. With the layout of a standard English-language keyboard Ctrl+& is directly to the right of Ctrl+^ while Ctrl+4 is directly to the right of Ctrl+_.
Ways to enter some common modifiers using control keys.
Here is

.
| Out[4]= |  |
You can use

as a variable.
| Out[5]= |  |