Change indent in standard table of content (not tocloft)

Question

I know that the indenting size of chapter, section and subsection can be changed in tocloft package using \cftsetindents. I am using a Latex template and I shouldn't use tocloft or any other TOC packages. So my question is how can I control those indents in the standard TOC? it's okay if I need to modify the cls files, etc but I don't know what are the settings to be changed.

\documentclass[singlespace,tocchapterhead,a4paper,openright,twoside]{MyThesisEnglish}
\usepackage{paralist}
\usepackage{natbib}
\usepackage[figuresright]{rotating}
\usepackage{lscape}
\usepackage[caption=false]{subfig}
\usepackage{graphicx}
\usepackage{longtable}
\usepackage{calc}
\usepackage{mathrsfs}
\usepackage{stmaryrd}
\usepackage{eucal}
\usepackage{float}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{epstopdf}
\usepackage{epsfig}
\usepackage{setspace}
\usepackage{color}
\usepackage{epstopdf}
\usepackage{dsfont}
\usepackage{algorithmic}
\usepackage{textcomp}
\usepackage{booktabs}
\usepackage{multirow}
\usepackage{url}
\usepackage{tipa}
\usepackage{array}
\citestyle{unsrt}
\usepackage[top=25mm, bottom=25mm, left=40mm, right=25mm]{geometry}


\begin{document}
\pagenumbering{roman}


\include{Title}


\addcontentsline{toc}{chapter}{\textbf{DEDICATIONS}}
\include{Dedication}


\addcontentsline{toc}{chapter}{\textbf{ABSTRACT}}
\include{Abstract}


\addcontentsline{toc}{chapter}{\textbf{\textit{ABSTRACT2}}}
\include{ABSTRACT2}


\addcontentsline{toc}{chapter}{\textbf{ACKNOWLEDGEMENTS}}
\include{Acknow}


\addcontentsline{toc}{chapter}{\textbf{APPROVAL}}
\include{Approval}


\addcontentsline{toc}{chapter}{\textbf{DECLARATION}}
\include{Declaration}


\makeatletter
\renewcommand{\@dotsep}{10000}
\makeatother


\tableofcontents
\listoftables
\listoffigures
\newpage
\addcontentsline{toc}{chapter}{\textbf{LIST OF ABBREVIATIONS}}
\include{abbrev}


\mainmatter
\singlespace


\addtocontents{toc}{\addvspace{.5cm}}
\include{Chapter1}
\addtocontents{toc}{\addvspace{.5cm}}
\include{Chapter2}
\addtocontents{toc}{\addvspace{.5cm}}
\include{Chapter3}
\addtocontents{toc}{\addvspace{.5cm}}
\include{Chapter4}
\addtocontents{toc}{\addvspace{.5cm}}
\include{Chapter5}
\addtocontents{toc}{\addvspace{.5cm}}
\include{Chapter6}


\addcontentsline{toc}{chapter}{\textbf{REFERENCES/Bibliography}}
\begin{singlespace}
\bibliography{bibfile}
\bibliographystyle{ai}
\end{singlespace}

\newpage
\addcontentsline{toc}{chapter}{\textbf{APPENDICES}}
\appendix
\include{AppendixA}
\include{AppendixB}


\addcontentsline{toc}{chapter}{\textbf{BIODATA OF STUDENT}}
\include{biodata}                                                %
\addcontentsline{toc}{chapter}{\textbf{LIST OF PUBLICATIONS}}
\include{listofpublications}                                    %
\end{document}

Update: I increased the indention for chapter titles using \setlength\@tempdima{5.5em} but still couldn't find the way to increase the indention of sections and subsections. https://imgur.com/a/3lUcG

\renewcommand*\l@chapter[2]{%
  \ifnum \c@tocdepth >\m@ne
\setcounter{secnumdepth}{3}%
\setcounter{tocdepth}{2} %   
    \addpenalty{-\@highpenalty}%
    \vskip .1cm \@plus \p@ %{the v-distance between the headings in the contents}
    \setlength\@tempdima{5.5em}% %{the distance between the number of the chapter and its title}
    \begingroup
           \parindent\z@ \rightskip \@pnumwidth
            \parfillskip -\@pnumwidth
      \leavevmode
      \advance \leftskip \@tempdima 
      \hskip  -\leftskip 
      \etchapterheadstyle{#1}\nobreak
% CCW: The following 3 lines add dots to the chapter TOC listings
      \leaders\hbox{$\m@th
        \mkern \@dotsep mu\hbox{.}\mkern \@dotsep
        mu$}\hfill
      \hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
      \penalty\@highpenalty
    \endgroup
  \fi} 

Answer 1

The ToC entries for \chapters, \sections and \subsections are handled by \l@chapter, \l@section and \l@subsection, respectively. Here are the definitions of each (from report.cls):

\newcommand*\l@chapter[2]{%
  \ifnum \c@tocdepth >\m@ne
    \addpenalty{-\@highpenalty}%
    \vskip 1.0em \@plus\p@
    \setlength\@tempdima{1.5em}%
    \begingroup
      \parindent \z@ \rightskip \@pnumwidth
      \parfillskip -\@pnumwidth
      \leavevmode \bfseries
      \advance\leftskip\@tempdima
      \hskip -\leftskip
      #1\nobreak\hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
      \penalty\@highpenalty
    \endgroup
  \fi}
\newcommand*\l@section{\@dottedtocline{1}{1.5em}{2.3em}}
\newcommand*\l@subsection{\@dottedtocline{2}{3.8em}{3.2em}}

Within \l@chapter, the length \@tempdima specifies the box width that holds the chapter number. The default definition, as you can see above, is 1.5em. You can change this using the following

\usepackage{etoolbox}
\makeatletter
% \patchcmd{<cmd>}{<search>}{<replace>}{<success>}{<failure>}
\patchcmd{\l@chapter}{1.5em}{5.5em}{}{}
\makeatother

to increase the box width to 5.5em, say.

Note how \l@section's definition also uses 1.5em as the first length parameter, followed by 2.3em. The first length is the indentation, while the second length is the width of the box containing the section number. The first length in \l@subsection is 3.8em (or 1.5em + 2.3em) and puts its number in a box of width 3.2em. So it seems the indentation of the sub-level is the sum of the indentation of its parent level and the parent's box width. With this is mind, \l@subsubsection should have an indentation of 7em (3.8em + 3.2em):

\newcommand*\l@subsubsection{\@dottedtocline{3}{7.0em}{4.1em}}

Look at that!

You can change the \section and \subsection indentation to now coincide with your 5.5em \l@chapter number-box width using

\makeatletter
\patchcmd{\l@section}{5.5em}{2.3em}{}{}% Since \l@chapter uses 5.5em for \@tempdima
\patchcmd{\l@subsection}{7.8em}{3.2em}{}{}% Since \l@section uses 5.5em + 2.3em
\makeatletter