This one is probably straightforward, but I'm not missing something with the newtheorem environment and using newcounter. I would like a new environment on its own counter like "Environment M.1", but I get "Environment M.0.1" in the following.
\documentclass{article}
\newcounter{mycounter}
\renewcommand{\themycounter}{M.\arabic{mycounter}}
\newtheorem{myenv}{Environment}[mycounter]
\begin{document}
\begin{myenv}[Some Stuff]
Here are some comments.
\end{myenv}
\end{document}
You don't need to define a new counter -- \newtheorem{myenv} already creates the necessary counter myenv. Just redefine \themyenv.
\documentclass{article}
\newtheorem{myenv}{Environment}
\renewcommand*{\themyenv}{M.\arabic{myenv}}
\begin{document}
\begin{myenv}[Some Stuff]
Here are some comments.
\end{myenv}
\end{document}
Some explanation what went wrong: \newtheorem{myenv}{Environment}[mycounter] tells LaTeX to number the theorem environment myenv “within” mycounter. This means that \themyenv is set to \themycounter.\arabic{myenv} and the myenv counter is reset whenever mycounter is incremented. The typical use of this is to number within sections or chapters. On the other hand, \newtheorem{myenv}[mycounter]{mycounter} tells LaTeX to use mycounter to number myenv.
