\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
a=b
\end{equation*}
\begin{equation}
a=b
\end{equation}
\begin{equation}\label{xx}
\begin{split}
a& =b+c-c\\
 & \quad +e-f\\
 & =g+h\\
 & =i
\end{split}
\end{equation}
\begin{multline}
a+b+c+d+e+f\\
+i+j+k+l+m+n
\end{multline}
\begin{gather}
a_1=b_1+c_1\\
a_2=b_2+c_2-d_2+e_2
\end{gather}
\begin{align}
a_1& =b_1+c_1\\
a_2& =b_2+c_2-d_2+e_2
\end{align}
\begin{align}
a_{11}& =b_{11}&
  a_{12}& =b_{12}\\
a_{21}& =b_{21}&
  a_{22}& =b_{22}+c_{22}\\
\end{align}
\begin{flalign}
a_{11}& =b_{11}&
  a_{12}& =b_{12}\\
a_{21}& =b_{21}&
  a_{22}& =b_{22}+c_{22}\\
\end{flalign}

Sub-numbered equations
\begin{subequations}
\begin{equation}N = \theequation\end{equation}
\begin{equation}N = \theequation\end{equation}
\end{subequations}

Subnumbered, but some unnumbered (before/after) or  tagged explicitly (before/after)
\begin{subequations}
\begin{equation}N = \theequation\end{equation}
\begin{equation}\nonumber N = \theequation\end{equation}
\begin{equation} N = \theequation\nonumber\end{equation}
\begin{equation}\tag{\theequation$'$}N = \theequation\end{equation}
\begin{equation}N = \theequation\tag{\theequation$''$}\end{equation}
\begin{equation}N = \theequation\end{equation}
\end{subequations}

\end{document}