starting new paragraph with \marginpar that is correctly aligned
Add \leavevmode so the paragraph will be started anyhow.
\documentclass[a4paper,11pt]{scrartcl}
\usepackage[utf8]{inputenc}
\newcommand{\introduce}[1]{%
\leavevmode % if at the start of a paragraph
\marginpar{#1}% the note
\emph{#1}% the text
}
\begin{document}
Bacon ipsum dolour sit amet porchetta beef turkey, bacon turducken
boudin hamburger venison ball tip. Brisket pork loin bresaola short
loin ground round leberkas pastrami tongue jerky cow turducken beef
ribs. Pork ribeye landjaeger prosciutto pig venison tenderloin.
Swine beef ribs kielbasa, porchetta tenderloin salami venison pork
belly tail.
\introduce{More Bacon} ipsum dolour sit amet porchetta beef turkey,
bacon turducken boudin hamburger venison ball tip. Brisket pork loin
bresaola short loin ground round leberkas pastrami tongue jerky cow
turducken beef ribs. Pork ribeye landjaeger prosciutto pig venison
tenderloin. Swine beef ribs kielbasa, porchetta tenderloin salami
venison pork belly tail.
Ipsum dolour sit amet porchetta beef turkey, bacon turducken boudin
hamburger \introduce{even more Bacon} venison ball tip. Brisket pork
loin bresaola short loin ground round leberkas pastrami tongue jerky
cow turducken beef ribs. Pork ribeye landjaeger prosciutto pig venison
tenderloin. Swine beef ribs kielbasa, porchetta tenderloin salami
venison pork belly tail.
\end{document}
\documentclass[a4paper,11pt]{scrartcl}
\usepackage[utf8]{inputenc}
\newcommand{\introduce}[1]{\mbox{}\marginpar{#1}\emph{#1}}
\begin{document}
Bacon ipsum dolour sit amet porchetta beef turkey, bacon turducken boudin hamburger venison ball tip. Brisket pork loin bresaola short loin ground round leberkas pastrami tongue jerky cow turducken beef ribs. Pork ribeye landjaeger prosciutto pig venison tenderloin. Swine beef ribs kielbasa, porchetta tenderloin salami venison pork belly tail.
\introduce{More Bacon} ipsum dolour sit amet porchetta beef turkey, bacon turducken boudin hamburger venison ball tip. Brisket pork loin bresaola short loin ground round leberkas pastrami tongue jerky cow turducken beef ribs. Pork ribeye landjaeger prosciutto pig venison tenderloin. Swine beef ribs kielbasa, porchetta tenderloin salami venison pork belly tail.
Ipsum dolour sit amet porchetta beef turkey, bacon turducken boudin hamburger \introduce{even more Bacon} venison ball tip. Brisket pork loin bresaola short loin ground round leberkas pastrami tongue jerky cow turducken beef ribs. Pork ribeye landjaeger prosciutto pig venison tenderloin. Swine beef ribs kielbasa, porchetta tenderloin salami venison pork belly tail.
\end{document}
