Latex: how to break the line in multirow inside the tabular

p column and \parbox also works:

\usepackage{multirow}

\begin{document}
\begin{center}
\begin{tabular}{|p{1.5cm}|l|l|l|l|l|}
    \hline
    \multirow{2}{*}{\parbox{1.5cm}{Long text to break}}
    & Thing  & \multicolumn{2}{|c|}{Thing 2} & \multicolumn{2}{|c|}{Thing 3} \\
    \cline{2-6}
    & sth 1 & sth 1 & sth 2 & sth 1  & sth 2 \\ 
    \hline
    \hline
\end{tabular}
\end{center}
\end{document}

You could try to minipage it:

\begin{center}
\begin{tabular}{|l|l|l|l|l|l|}
    \hline
    \multirow{2}{*}{\begin{minipage}{0.5in}Long text to break\end{minipage}}
    & Thing  & \multicolumn{2}{|c|}{Thing 2} & \multicolumn{2}{|c|}{Thing 3} \\
    \cline{2-6}
    & sth 1 & sth 1 & sth 2 & sth 1  & sth 2 \\ 
    \hline
    \hline
\end{tabular}
\end{center}

---

However, in your particular case, my suggestion would simply be to loosen the restrictions of the other columns, because there is too much space wasted there. With each p{}, that forces the other columns to be a certain width, so there is not enough room for the first column.

The following code looked presentable to me when I compiled it:

\begin{center}
\begin{tabular}{|l|l|l|l|l|l|}
    \hline
    \multirow{2}{*}{Long text to break}
    & Thing  & \multicolumn{2}{|c|}{Thing 2} & \multicolumn{2}{|c|}{Thing 3} \\
    \cline{2-6}
    & sth 1 & sth 1 & sth 2 & sth 1  & sth 2 \\
    \hline
    \hline
\end{tabular}
\end{center}

for me the shortest and most practical answer:

use \linewidth as the length for the {width} parameter.

\usepackage{multirow}
\begin{document}

\begin{center}
\begin{tabular}{|p{1cm}|p{2.5cm}|p{2cm}|p{2cm}|p{2cm}|p{2cm}|}
\hline
\multirow{2}{\linewidth}{Long text to break} % HERE IS A PROBLEM
    & Thing  & \multicolumn{2}{|c|}{Thing 2} & \multicolumn{2}{|c|}{Thing 3}    
\\ \cline{2-6}
    & sth 1 & sth 1 & sth 2 & sth 1  & sth 2 \\ \hline
\hline
\end{tabular}
\end{center}

\end{document}

That's it!

The only possible problem is that in the improbable case that the text in the other cells is really short it may look like that:

However if typically your table has more text on the other cells than just "sth1" it will look great:

For me it worked to use the build-in command of "multirow" - the {*} is "{width}"