\documentclass{article}
\usepackage{todonotes}

\begin{document}

Hello World

\section{First Section}

\subsection{test 1.1}

\todo[inline]{This is a TODO.
}

\subsection{test 1.2}

\section{Second Section}

\input{sub.tex}

\subsection{test 2.2}

\subsection{test 2.3}

\end{document}
