\documentclass{article}
%\usepackage{amsmath}
\begin{document}
\section{Regular usage}

Just\\
line break.

Brackets\\[1em]
on the same line.

Star\\*
on the same line.

Star and brackets\\*[1em]
on the same line.

\begin{tabular}{l}
tabular\\
\hline
Two\\
rows\\
\hline
Brackets \\[1em]
in the same row\\[1pt]
\hline
Star \\*
in the same row\\*
\hline
Start and brackets\\*[1em]
in the same row\\*[1pt]
\hline
\end{tabular}

\section{Problems}
Brackets and starts disappear in the beginning of
lines and table rows.

Brackets\\
[1em] on the next line.

\begin{tabular}{l}
tabular\\
\hline
Brackets \\
[1em] in the next row\\
[1pt]
\hline
\end{tabular}

\section{Problems when usepackage\{amsmath\} absent}

Star\\
* on the next line.

Star and brackets\\
*[1em] on the next line.

\begin{tabular}{l}
tabular\\
\hline
Star\\
* in the next row. \\
\hline
Star and brackets\\
*[1em] in the next row. \\
\hline
\end{tabular}
\end{document}