Whenever an expression is too long, it will be broken automatically in inline mode but done nothing in displaymode
E.g.
MWE:
\documentclass[11pt]{article}
\begin{document}
$1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0$
\[1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0\]
\end{document}
The upper one will be broken whereas the lower one not.
So how to make auto break also works in displaymode?
breqnbreaks automatically. However the normally you have to break manually. – Marco Daniel Mar 10 '13 at 16:42