%%
%% The LaTeX Companion, 3ed
%%
%% Example A-5-3 on page II-690 in "ifthen --- Advanced control structures".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exareport}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{111.0pt}
\setcounter{chapter}{1}
\setcounter{section}{5}
% provide some values for the example:

%StartShownPreambleCommands
\usepackage{ifthen}
\newcommand\toEng[1]{\arabic{#1}\textsuperscript{%
 \ifthenelse{\value{#1}=1}{st}{%
  \ifthenelse{\value{#1}=2}{nd}{%
   \ifthenelse{\value{#1}=3}{rd}{%
    \ifthenelse{\value{#1}<20}{th}}%
                              {\typeout{Value too high}}}}}}
%StopShownPreambleCommands

\begin{document}
This is the \toEng{section} section in the \toEng{chapter}
appendix.
\end{document}