New Theorem Style

Question

I'm pretty new to LaTeX. And I don't really understand the following code, it is merely a pasted template from wikipedia.

I'm trying to make a simple theorem style which is identical to the normal theorem style but has a dash after the number. I tried this but I keep getting an error on and off about numbering. As I'm typing it has stopped for a while. Anyway, what is the best way to tell it to do the theorem style? I feel there's probably a very simple way.

\newtheoremstyle{theoremdd}% name of the style to be used
{}% measure of space to leave above the theorem. E.g.: 3pt
{}% measure of space to leave below the theorem. E.g.: 3pt
{}% name of font to use in the body of the theorem
{}% measure of space to indent
{}% name of head font
{}% punctuation between head and body
{ }% space after theorem head; " " = normal interword space
{\thmname{#1}\thmnumber{ #2'}\thmnote{ (#3)}.}

\theoremstyle{theoremdd}
\newtheorem{thmd}{Theorem}[section]

...

\begin{thmd}[hello]
 jfgh
\end{thmd}

Whilst it would be nice to understand what all the code means, I just desperately need a fix.

Thanks, Richard

Edit/update:

Apart from completely not understanding how the LaTeX code works, I have a working product, but it was completely a workaround. Correct me if I'm wrong, but the #2 represents a number. All I want to do it subtract 1 from the counter. Currently this is my fix:

 \newtheoremstyle{theoremd}% name of the style to be used
  {}% measure of space to leave above the theorem. E.g.: 3pt
  {}% measure of space to leave below the theorem. E.g.: 3pt
  {\em}% name of font to use in the body of the theorem
  {}% measure of space to indent
  {\bf}% name of head font
  {.}% punctuation between head and body
  { }% space after theorem head; " " = normal interword space
  {\thmname{#1}\thmnumber{\addtocounter{thm}{-1} #2}'\thmnote{\textnormal{ (#3)}}}

As you can see I'm clueless. More understanding would be greatly appreciated.

Latest edit:

This is what I've settled for. Comments would definitely be appreciated. I'm still unsure about how it all works but I've learned a lot thanks to you all.

\newtheoremstyle{theoremd}% name of the style to be used
{}% measure of space to leave above the theorem. E.g.: 3pt
{}% measure of space to leave below the theorem. E.g.: 3pt
{\em}% name of font to use in the body of the theorem
{}% measure of space to indent
{\bf}% name of head font
{.}% punctuation between head and body
{ }% space after theorem head; " " = normal interword space
{\thmname{#1}\thmnumber{\addtocounter{thm}{-1} #2$^\prime$}\thmnote{\textnormal{ (#3)}}}

Answer 1

Just fill in the necessary parameters.

\documentclass{article}
\usepackage{amsthm}

\newtheoremstyle{theoremdd}% name of the style to be used
  {\topsep}% measure of space to leave above the theorem. E.g.: 3pt
  {\topsep}% measure of space to leave below the theorem. E.g.: 3pt
  {\itshape}% name of font to use in the body of the theorem
  {0pt}% measure of space to indent
  {\bfseries}% name of head font
  {. ---}% punctuation between head and body
  { }% space after theorem head; " " = normal interword space
  {\thmname{#1}\thmnumber{ #2}\textnormal{\thmnote{ (#3)}}}

\theoremstyle{theoremdd}
\newtheorem{thmd}{Theorem}[section]

\begin{document}

\section{Whatever}

\begin{thmd}
This is a test theorem.
\end{thmd}

\end{document}

Answer 2

So you want a Bourbaki style layout? It's easy to do with the ntheorem package:

\documentclass{book}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[small]{titlesec}
\titleformat{\section}{\normalsize\bfseries}{\thesection.}{0.5em}{}

\usepackage[ thmmarks, thref]{ntheorem}
\usepackage{chngcntr}
\usepackage{cleveref}

\renewcommand\thesection{\arabic{section}}

\theoremstyle{plain}
\theoremheaderfont{\scshape}
\theoremseparator{.~---}
\newtheorem{thm}{Theorem}
\counterwithin*{thm}{chapter}

\pagestyle{plain}

\begin{document}

\section{A first section}
\begin{thm}\label{testthm}
This is a test theorem. 
\end{thm}
We see in \cref{testthm}\dots

\end{document}