\documentclass{article}
\usepackage{etoolbox}
\usepackage{amsmath}
\def\addrow#1{\text{#1}\\}
\newcommand{\annot}[1]{%
\raisebox{\dimexpr-\height+\ht\strutbox\relax}{$%
\left[\begin{array}[c]{l}%
\forcsvlist\addrow{\strut #1}%
\end{array}\right]$}}
\begin{document}
\annot{This,Det,Dem,} is a\rlap{\rule{4cm}{.4pt}}\llap{\rule{4cm}{.4pt}} sentence \annot{with,P} some annotations.
\end{document}