\theequation, that counter is stopped and an
\alpha extension is added. When you turn off subequation
numbering, things go back to what you want them to go back to...
To use subequations, first you must put the style file where LaTeX can find it. Who knows, you might already have it. Add this option to the list in the inputs:
\documentstyle[...,subeqn,...]{style}
Yeah, I know, it's only 209 and not 2e.
Here's a little example of how to code in subequations:
...and important pair of equations blah blah blah:
\begin{subequations}
\begin{equation}
\rho(r)=\dispfrac{\sigma^2}{2\pi G r_c^2}
\dispfrac{1}{1+\left(\frac{r}{r_c}\right)^2}
\label{rho-PID-eqn}
\end{equation}
\begin{equation}
M_{PID}(R) = \dispfrac{\sigma^2}{G}
2\left[ R - r_c \arctan\left(\dispfrac{R}{r_c}\right)\right]
\label{M-PID-eqn}
\end{equation}
\end{subequations}
And now back to the regular text...
The output looks like this:
\label{} just after the
\begin{subequations} line, references will be to the
collection of equations, (4) in the example above. Or, like the example
code, you can reference each equation individually.
subeqn.sty merely does all the counter resetting stuff
in a nice robust way. It's not a heavy-duty environment like
equation or figure. So you can add any amount
of text, figures, whatever, within the scope of the
subequations environment. All that happens is your equations
are numbered differently.
subfig.sty
does the same numbering trick, but with Figures. It works just the
same. Turn it on \begin{subfigures} when you want to start
the sub-numbering, plop in the figures, and
turn it off \end{subfigures} when you're done. Just like
subequations, you can have text and other things inside the
subfigures environment. Only the figure numbering will
change. By the way, subfig.sty is just
subeqn.sty with a global search-and-replace "equation" -->
"figure", so I can't take any credit for it whatsoever.