# Formatting subequations where one of them is a piece-wise function

I have the following LaTeX code:

\begin{subequations}
\begin{align}
F_h(t) = F(\Delta y) = \left\{
\begin{array}{lr}
K_h (\Delta y)^{P_h} \; \text{if} \; \Delta y > 0\\
0 \; \; \hspace{1.23cm}\; \text{if} \; \Delta y \leq 0
\end{array}
\right. \label{eqn: hammer1}\\
F_h(t) = -m_h \label{eqn: hammer2}
\end{align}
\end{subequations}


when I compile it, the 2nd equation is aligned to the right-most margin of equation 1. How do I set it such that the 2nd equation is aligned to the left-most margin of the first equation?

You forgot to place the alignment character inside your align construction. Also, consider using cases for the conditional constructions:

\documentclass{article}
\usepackage{mathtools}% http://ctan.org/pkg/mathtools

\begin{document}

\begin{subequations}
\begin{align}
F_h(t) &= F(\Delta y) = {\begin{cases}
K_h (\Delta y)^{P_h} & \text{if $\Delta y > 0$} \\
0                    & \text{if $\Delta y \leq 0$}
\end{cases}}
\label{eqn: hammer1} \\
F_h(t) &= -m_h \label{eqn: hammer2}
\end{align}
\end{subequations}

\end{document}