Q: Simple question about mathmode Good morning, 
I would like to write a mathematical equation like:
$F_{i}$ 

but I want to shift down the letter i more than the effect of the command $_$. 

A: A quick fix: use an empty superscript along with the subscript. Compare the following.
\documentclass[12pt,a4paper]{article}
\usepackage{amsmath}
\begin{document}

$F_{i} F^{}_{i}$

\end{document}


Lowering the subscript gives the optical illusion that it also shifted to the right, to make this less obvious, I agree on the suggestion by @Bernard to use \mkern-1.5mu. See a comparison $F^{}_{i}$ vs. $F^{}_{\mkern-1.5mu i}$:
 
