While comparing the equations in the align* environment using the & sign before the = sign, it is giving error "! Misplaced alignment tab character &" [duplicate]

Question

\documentclass {article}
\usepackage {geometry}
\usepackage {amsmath}
\begin {document}
With align* command :-\\
\\
\\
\begin {align*}
a = b + c
\\ abcd = f + g + h
\end {align*}\\
\\
\\
\\
Without align* command :-\\
\\
\\
a = b +c
\\abcd = f + g + h
\\
\\
\\
With align* command and ''and'' symbol used:-\\
\\
\\
\begin {align*}
a & = b + c
\\ abcd & = f + g + h
\end {align*}\\
\\
\\
\\
Without align* command and `'and'' symbol used :-\\
\\
\\
a &= b +c
\\abcd &= f + g + h
\\
\\
\\
\end {document}