Why does indenting a paragraph compound on subsequent paragraphs?

Question

I'm attempting to indent cases and subcases of a proof by 1.3cm. However, when I apply adjustwidth to a subcase, it seems to affect the subcase following it.

Here is how it looks with my current code:

And here's how I'd like it to look:

Code I'm using (excuse the sloppy reuse of code):

\documentclass{article}
\usepackage{amsthm}
\usepackage{changepage}
\begin{document}
    \begin{proof}
        {\it Case 1.} First case.

        \begin{adjustwidth}{1.3cm}{}
            Case explanation.
        \end{adjustwidth}

        {\it Case 2.} Second case.

        \begin{adjustwidth}{1.3cm}{}
            {\it Case 2.1.} First part.

            {\it Case 2.2.} Second part.

            \begin{adjustwidth}{1.3cm}{}
                Second part explanation.
            \end{adjustwidth}
        \end{adjustwidth}
    \end{proof}
\end{document}

How can I adjust my code to allow uniform indenting of sections? If it's not possible with adjustwidth, what can I use?

Answer 1

The following code will yield the desired output however as said in the comments of your question you might want the review some of your code in order to remove the \it commands and replace them by {\itshape } for instance

\documentclass{article}

\usepackage{amsthm}
\usepackage{changepage}

\begin{document}

\begin{proof}
    \textit{Case 1.} First case.

    \begin{adjustwidth}{1.3cm}{}
        Case explanation.
    \end{adjustwidth}

    \textit{Case 2.} Second case.

    \begin{adjustwidth}{1.3cm}{}
        \textit{Case 2.1.} First part.\\
        \textit{Case 2.2.} Second part.

        \begin{adjustwidth}{1.3cm}{}
            Second part explanation.
        \end{adjustwidth}
    \end{adjustwidth}
\end{proof}

\end{document}

The "bad" indenting came from the new paragraph introduced by the empty line between Case 2.1 and Case 2.2. This can be visualised if you add \setlength{\parindent}{0pt} in your preamble. Then the "bad" indent is still there but is of 0pt which gives the desired output.

Answer 2

You should almost never need manual numbering, indentation or \\ in a LaTeX document. If you let the system do this it is much easier to handle cross references, and get consistent layouts, or to change the layouts without editing the document body.

Here I use some basic settings in enumitem to save setting the lower level latex list parameters directly. The package has a lot of parameters so the exact indentation style can be customised more without changing the document markup.

\documentclass{article}

\usepackage{amsthm}

\usepackage{enumitem}

\newlist{pcases}{enumerate}{3}
\setlist[pcases]{label=\textit{Case \arabic*},leftmargin=*}
\setlist[pcases,2]{label=\textit{\thepcasesi.\arabic*}}

\begin{document}

\begin{proof}\mbox{}
    \begin{pcases}
    \item First case.

        Case explanation.

    \item Second case.
        \begin{pcases}
        \item First part.
        \item Second part.

            Second part explanation.
        \end{pcases}
    \end{pcases}
\end{proof}

\end{document}