I managed to either wrap text in latex using
\newcolumntype{L}{>{\centering\arraybackslash}m{3cm}}
or merge rows using
\multirow{3}{*}{Problem}
But I cannot use these two together. I would like a table like this, where "Problem x" and "Subproblem x.y" is centered both horizontally and vertically.
+-----------+----------------+---------------------------+
| | SUBPROBLEM 1.1 | This line is long and too |
| | | long to fit in one line. |
| PROBLEM 1 |----------------+---------------------------+
| | SUBPROBLEM 1.2 | This could also be long |
| | | and it is. |
+-----------+----------------+---------------------------+
| | SUBPROBLEM 2.1 | This could also be long. |
| PROBLEM 2 |----------------+---------------------------+
| | SUBPROBLEM 2.2 | This one is not long. |
+-----------+----------------+---------------------------+
Minimal working problem.
\documentclass{article}
\usepackage{array, multirow}
\newcolumntype{L}{>{\centering\arraybackslash}m{3cm}}
\begin{document}
\begin{tabular}{|c|c|L|}
\hline
\multirow{3}{*}{P1}
& P1.1 & This is indeed a problem. \\\hline
& P1.2 & Also this.\\\hline
& P1.3 & This migth also be one.\\\hline
\end{tabular}
\end{document}

