Aligning equations with align

Question

Imagine I have the following system

\begin{align}
    a&=b\\
    abc&=def+1\\
    e&=mc^2-H(x,y,z)
\end{align}

I want to add a first column aligned in a different manner, like so

is this possible to do with align? If not, what good alternative could I use? Note that I want to index each equation separately, so I believe matrix wouldn't work.

Answer 1

You asked in a follow-up comment:

Do you know if it is somehow possible to control the spacing between the left-hand column of equations (m's) and the right-hand one?

That's actually where the alignat and alignat* environments very much come into their own. Consider the following three alignat*{2} environments:

The first group shows that, in general, it's not a good idea to provide no separation instruction between the left-hand and right-hand equations. The second and third group differ only in where the instruction \qquad is placed: it's in row 2 for the middle group, and in row 1 for the third and final group. In practice, one would (should?) place the spacing instruction (here: \qquad) in the row for which the spacing is narrowest to begin with.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{alignat*}{2}
m&=0{:} &        a&=b\\
m&=1{:} &        abc&=def+1\\
m&=2{:} &        e&=mc^2-H(x,y,z)
\end{alignat*}

\begin{alignat*}{2}
m&=0{:} &        a&=b\\
m&=1{:} &\qquad  abc&=def+1\\
m&=2{:} &        e&=mc^2-H(x,y,z)
\end{alignat*}

\begin{alignat*}{2}
m&=0{:} &\qquad  a&=b\\
m&=1{:} &        abc&=def+1\\
m&=2{:} &        e&=mc^2-H(x,y,z)
\end{alignat*}
\end{document}