Amsmath, spacing, left alignment

Question

It was recommended that I include amsmath package to use \tag{} (which works). When I do this however, it somehow changes my predefined spacing; I would like everything to be left aligned, no left spacing for beginning of paragraphs, single line space between all equations, all paragraphs.

Sample code below; if you comment out amsmath line, you'll see the output I was aiming for. My question is what additional commands do I use for amsmath to this kind of output?

\documentclass[12pt,fleqn]{article}
\usepackage{amsmath}
\setlength{\parindent}{0pt}
\usepackage{graphicx}
\usepackage{cancel}
\usepackage{listings}
\usepackage[latin5]{inputenc}
\usepackage{color}
\setlength{\parskip}{8pt}
\setlength{\parsep}{0pt}
\setlength{\headsep}{0pt}
\setlength{\topskip}{0pt}
\setlength{\topmargin}{0pt}
\setlength{\topsep}{0pt}
\setlength{\partopsep}{0pt}
\setlength{\mathindent}{0cm}

\begin{document}

Some line 

\[ y = x^2 \]

Another line

\[ y = x^2  \]

\[ y = x^2 \]

One more line

\end{document}

Answer 1

As others have mentioned you really should not be doing this. That being said, the code below provides a hack of the equation* environment which is what \[ ... \] construct becomes after you load amsmath. I am not able to see any difference in your MWE's output with amsmath loaded and this hack applied. No guarantee that there won't be other cases where the results with amsmath will vary though.

\usepackage{amsmath}
\makeatletter
\renewenvironment{equation*}{\vspace{\dimexpr -2\baselineskip+2.5pt\relax}\belowdisplayskip=0pt%
  \mathdisplay@push
  \st@rredtrue \global\@eqnswfalse
  \mathdisplay{equation*}%
}{%
  \endmathdisplay{equation*}%
  \mathdisplay@pop
  \ignorespacesafterend%
}
\makeatother

Answer 2

You must never have blank lines above a displayed equation (unless it really starts a paragraph, which is bad style anyway); a blank line below can be used, but only if the equation ends a paragraph.

For consecutive displayed equations use the gather* environment.

\documentclass[12pt,fleqn]{article}
\usepackage[latin5]{inputenc}
\usepackage{amsmath}

\setlength{\parindent}{0pt}
\usepackage{graphicx}
\usepackage{cancel}
\usepackage{listings}
\usepackage{color}
\setlength{\parskip}{8pt}
\setlength{\parsep}{0pt}
\setlength{\headsep}{0pt}
\setlength{\topskip}{0pt}
\setlength{\topmargin}{0pt}
\setlength{\topsep}{0pt}
\setlength{\partopsep}{0pt}
\setlength{\mathindent}{0cm}

\begin{document}

Some line 
\[
y = x^2
\]
Another line
\begin{gather*}
y = x^2\\
y = x^2
\end{gather*}
One more line

\end{document}