\newcommand and \def not working

Please provide complete code example you are using, with the tags ([preamble]…[/preamble]), etc, so that situation can be reproduced.

Preamble:

%PREAMBLE:

\textwidth 6.5in
\textheight 9.0in
\voffset=-.8in
\hoffset=-.5in

\usepackage[T1]{fontenc}
\usepackage{mathptmx}
\usepackage{graphicx}
\usepackage{color}
\usepackage{amsmath}
\usepackage{xspace}
\usepackage{latexsym}
\usepackage[margin=10pt,width=3.0in,font=small,labelfont=bf]{caption}
\usepackage{natbib} \bibpunct[, ]{(}{)}{;}{a}{}{,}
\usepackage{esint}
%\usepackage{undertilde}
\def\undertilde#1{\mathord{\vtop{\ialign{##\crcr
   $\hfil\displaystyle{#1}\hfil$\crcr \noalign{\kern1.5pt\nointerlineskip}
   $\hfil\tilde{}\hfil$\crcr\noalign{\kern1.5pt}}}}}
\newcommand{\utilde}{\undertilde}% replaces undertilde for the mac
\usepackage{cancel}
\usepackage{float}
\usepackage{xcolor}
\usepackage{subfiles}
\usepackage{sidecap}
\sidecaptionvpos{figure}{c}
\usepackage{enumitem}
\usepackage{empheq}
\usepackage{hyperref}
\usepackage[toc,page]{appendix}
\usepackage{boxedminipage}
\usepackage{capt-of}
\usepackage[export]{adjustbox}
\usepackage{makeidx}
%\usepackage{showidx}
\usepackage[totoc]{idxlayout}

\newcommand{\s}{\, \text{s}}
\newcommand{\m}{\, \text{m}}
\newcommand{\ps}{\, \text{s}^{-1}}
\newcommand{\kv}{\kappa_v}

%\def\lc{\it}
\def\lc{\color{blue}}

\def\kh{\kappa_h}
\def\eps{\varepsilon}
\def\del { {\vec\nabla}}
\def\delt{\Delta_h}
\def\ni{\noindent}

\def\haf{\frac{1}{2}}

\def\vx{\vec{x}}
\def\vy{\vec{y}}
\def\vu{\vec{u}}
\def\vv{\vec{v}}
\def\vw{\vec{w}}
\def\vz{\vec{z}}
\def\vo{\vec{\omega}}
\def\di{\partial}
\def\ue{\utilde\epsilon}
\def\uA{\utilde{A}}
\def\uB{\utilde{B}}
\def\uC{\utilde{C}}
\def\uD{\utilde{D}}
\def\uI{\utilde{I}}
\def\udelta{\utilde{\delta}}
\def\ur{\utilde{r}}
\def\tk{\tilde{k}}
\def\hw{\hat{w}}
\def\hu{\hat{u}}
\def\hv{\hat{v}}
\def\hb{\hat{b}}
\def\hp{\hat{\pi}}

\def\prm{^\prime}
\def\prmm{^{\prime\prime}}
\def\prmmm{^{\prime\prime\prime}}
\def\prmmmm{^{\prime\prime\prime\prime}}

\def\sta{^\star}
\def\dag{^\dagger}
\def\limx{\lim_{|\Delta\vx |\rightarrow 0}~}

\newcommand{\ddt}[1]{\frac{\di{#1}}{\dt}}

% Abbreviations for unit vectors:
\newcommand{\he}[1]{\hat{e}^{(#1)}}
\newcommand{\hep}[1]{\hat{e}^{\prime(#1)}}

% Abbreviation for k'th derivative:
\newcommand{\fk}[1]{f^{(#1)}}

% These abbreviations are useful for beginning and ending displayed equations:
\newcommand{\beq}{\begin{displaymath}}
\newcommand{\eeq}{\end{displaymath}}
\newcommand{\beqn}{\begin{equation}}
\newcommand{\eeqn}{\end{equation}}
\newcommand{\be}{\begin{equation}}
\newcommand{\ee}{\end{equation}}

% Jeff added: -------------
\def\pder#1#2{\frac{\partial #1}{\partial #2}}
\def\md{\mathrm1gwpiim}
\def\oder#1#2{\frac{\md #1}{\md #2}}
%-------------------------------

\newcommand{\nl}{\newline}

\newcommand{\bmat}{\begin{quote}\color{blue}}
\newcommand{\emat}{\color{black}\end{quote}}

\newcommand{\mat}[1]{{\color{blue}\bf{#1}\color{black}}}
\newcommand{\rast}{{\color{red}*\color{black}}}

\makeindex

\makeatletter
\g@addto@macro\@floatboxreset\centering
\makeatother

%DOUBLESPACE
\renewcommand{\baselinestretch}{1.0}
\numberwithin{equation}{section}
\bibliographystyle{ametsoc}

\parskip=0.1in
\parindent = 0in

\newcommand{\toc}{\parskip=0in\tableofcontents\parskip=.1in}

\DeclareRobustCommand{\gobblefour}[4]{}
\newcommand*{\SkipTocEntry}{\addtocontents{toc}{\gobblefour}}

%\renewcommand{\theequation}{\thesection.\arabic{equation}}
                      %At end of section add \setcounter{equation}{0}
% My ''Example'' environment
\newcounter{exa}
\newenvironment{example}
{\begin{quote} \bf Example \stepcounter{exa}\arabic{exa}\rm:}{\end{quote}}

% My ''Exercise'' environment
\newcounter{exer}
\newenvironment{exercise}
{\bf Exercise \stepcounter{exer}\arabic{exer}\rm:}

%\newcommand{\sect}[1]{ \setcounter{equation}{0} \section{#1}}

\renewcommand{\arraystretch}{1.5}

Example equation:

\beq
\left[ {\begin{array}{*{20}{c}}
x  \\
y \\
\end{array}} \right] \quad\textrm{to}\quad
\left[ {\begin{array}{*{20}{c}}
y  \\
-x \\
\end{array}} \right].
\eeq

Wrap equation code with [latex]...[/latex]. QuickLaTeX detects only standard LaTeX environments, not custom defined. The ‘\beq’ will work fine if placed between the [latex]...[/latex].

Rely on standards environments, if you do not want to use [latex]...[/latex].

Ok, thank you for your quick replies! It still does not work, but I think I will just use the normal environments. That works, at least. I took this project over from someone else, so the newcommands don’t really matter to me.