double subscript error while compiling in italics mode

Question

I am a beginner in latex. I am getting double subscript error while compiling the eqn below. pls help

\textit{$[S_e]_e_=_1^N$} 

Answer 1

To have more than one letter in subscript, put the text in curly braces ({and }):

\documentclass[10pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\begin{document}

$[S_e]_{e=1}^N$

\end{document}

This gives:

Answer 2

An alternative, slightly aesthetic, to the answer of @Tom you can use \scriptscriptstyle in mathmode to decrease the subscript $e=1$. Thus N and e=1 have a bit more distance.

\documentclass[12pt]{article}
\usepackage{graphicx}
\begin{document}
$[S_e]_{\scriptscriptstyle e=1}^N$
\end{document}