To get an expression, exp, to appear as a subscript, you just type _{exp}.
Can be used only in math mode. Thus, for a simple expression that is part of the running text:
H$_2$O is the formula for water
should display as "H2O is the formula for water".
Note that the braces around the argument may be omitted if the
subscript is a single character.
If a symbol has both subscripts and superscripts, the order doesn't matter.
The following are equivalent:
a_n^2 and a^2_n
Subscripts are normally displayed in a smaller font; to prevent this and, for example, use a standard Roman font, one would use;
$J_{\rm Roman}$