Mathematica allows you to use any expression as a subscript.
Unless you specifically tell it otherwise, Mathematica will interpret a superscript as a power.
Special input forms based on control characters. The second forms given should work on any keyboard.
This enters a subscript using control keys.
In:= Expand[(1 + x_1+n)^4]
Just as ^ and _ go to superscript and subscript positions, so also & and = can be used to go to positions directly above and below. With the layout of a standard English-language keyboard & is directly to the right of ^ while = is directly to the right of _.
Ways to enter some common modifiers using control keys.
Here is .
In:= x &_
You can use as a variable.
In:= Solve[a^2 == %, a]
Ways to enter modifiers without control keys. All these forms can be used only inside \!\( ... \).