How to [import packages] or [define commands] into chapter files?

Question

I'd like to organize a thesis-like document with ut-thesis package such that there is a main .tex file as the following:

\documentclass{ut-thesis}

\degree{X}
\department{X}
\gradyear{X}
\author{X}
\title{X}

\setcounter{tocdepth}{2}

\flushbottom

\begin{document}

\begin{preliminary}

\maketitle

\begin{abstract}

\end{abstract}

\tableofcontents

\end{preliminary}

\include{chap1}

\addcontentsline{toc}{chapter}{Bibliography}
\bibliographystyle{plain}
\bibliography{thesis}

\end{document}

where chap1.tex is:

\chapter{Introduction}
This chapter is your first plan...

Now, assume that I want to define some commands or add some packages into my chapter file. For example, I may need to add amsthm package, in addition to the following commands:

\renewcommand{\qedsymbol}{$\blacksquare$}

\newtheorem{thm}{Theorem}
\newtheorem{lem}{Lemma}
\newtheorem{prop}[thm]{Proposition}
\newtheorem{cor}{Corollary}
\newtheorem{conj}{Conjecture}
\theoremstyle{definition}
\newtheorem{defn}{Definition}
\newtheorem{exmp}{Example}
\newtheorem{rem}{Remark} 

If I add anything before chapter{}, the following error will be raised:

Can be used only in preamble. \usepackage

How to import packages and commands into chapter files?

Answer 1

In general, one adds packages and defines macros common to the entire document in the preamble, which occurs after \documentclass and before \begin{document}, the start of the document environment.

One reason for this is the behavior of some packages that use \AtBeginDocument, which appends tokens to a token list that is expanded when the document environment begins, as explained here.

There are a few exceptions to this, but those exceptions are covered in the documentation and discussion about classes like memoir and maybe packages like cmap. In that case, one must use \RequirePackage, a topic that goes somewhat afield of this question here.

For the differences between \include and \input see this article. Again, you generally put common macros that you use document-wide into the preamble. That prevents an unexpected situation where you may be inside a scope (created by the grouping tokens { }, synonyms \bgroup ... \egroup, and additionally \begingroup ... \endgroup) and define a macro, etc. When you leave that scope, unless you used \global, that definition goes away. There are other caveats as well.

Doing things the usual way helps prevent unwanted errors, which can include not only the situation where document compilation halts, but also where compilation succeeds, yet subtle and unexpected errors might arise.