\documentclass[11pt]{article}
\begin{document}

\section{Example for paragraphs}

Some roman text

{\tt
should be typewriter font

should be typewriter font
}

\section{Example for itemize}

Another example

\begin{itemize}
\item one roman
{\tt
\item two typewriter
\item three typewriter
}
\begin{itemize}
\item subitem roman
\item subitem roman
\end{itemize}
\end{itemize}

Now a numbered list

\begin{enumerate}
\item one roman
{\tt
\item two typewriter
\item three typewriter
}
\begin{enumerate}
\item subitem roman
\item subitem roman
\end{enumerate}
\end{enumerate}

\section{Example for sections}
But sections are impervious!

\subsection{Title is roman}
Text is in roman.

{\tt
\subsection{Two in roman}
Text is in typewriter.

\subsection{Three in roman}
Text is in typewriter.
}
\end{document}
