How to create a custom subsection (with regular font size, bold letter, don't start a new line)

Question

I'd like to create a custom subsection that satisfies the followings:

• Bold letter
• The font size is the same as that of the paragraphs.
• Indent before the title
• Do not start a new line after the title.

Answer 1

This is easy to do with titlesec.

\documentclass{article}
\usepackage{lipsum}
\usepackage{titlesec}
\titleformat{\subsection}[runin]{\bfseries}{}{\parindent}{}[.]
\begin{document}
\section{A section}
\subsection{A subsection}
\lipsum[1-2]
\end{document}

I added a dot at the end of the subsection title because I thought it made sense to have a separation with the text. If you want to remove it or to change it, the part of the code responsible for that is the optional argument [.] of \titleformat.

Answer 2

You can use titlesec and specify \titleformat and \titlespacing.

I also suggest to add punctuation for run-in titles. You can make a period appear automatically, if you forget one, but not if the title already ends with punctuation.

\documentclass{article}
\usepackage{amsthm}% for \@addpunct
\usepackage{titlesec}
\usepackage{lipsum}
\makeatletter
\titleformat{\subsection}[runin]
  {\normalfont\bfseries}
  {\thesubsection}
  {0.5em}
  {}
  [@addpunct{.}]
\titlespacing{\subsection}
  {\parindent}
  {3.5ex plus 1ex minus 0.2ex}
  {0.5em}
\makeatother
\begin{document}
\section{A section}
\subsection{A subsection}
\lipsum[1][1-3]
\subsection{A subsection with period.}
\lipsum[2][1-3]
\subsection{Another subsection?}
\lipsum[3][1-3]
\lipsum[4][1-3]
\subsection*{Unnumbered!}
\lipsum[5][1-3]
\end{document}