breaking lines in a matrix/array environment
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[\begin{bmatrix*}[r]
a + b & e + f \\
{}+c + d & {}+g + h \\[6pt]
i + j & m + n \\
{}+k + l & {}+o + p \\
\end{bmatrix*}\]
\end{document}
To change an operator from unary to binary, put an empty group before it, e.g. {}+c+d
. You could use the \phantom
command to get the vertical alignment right: \phantom{{}+{}}a+b
see section 3.7.1 of the Not so short introduction to LaTeX.