\documentclass{amsart}
\begin{document}
\title{Title 1}
\maketitle
Please fill in all details here: \underline{\hspace{50cm}} \\
Thank you
\end{document}
I would like to make the underline several lines long, but as you can see it is running off the page here. (I put in the \\ because I want a line break space between the "detail line" and the "thank you" line)
\\as here, it makes TeX scream at you about badness 10000 which is as bad as it can get. – David Carlisle Aug 29 '16 at 16:10\\at the end of a paragraph as that is always just completely wrong so you do not want anything equivalent. If you are looking for a style with vertical space between paragagraphs (which is not what\\does) then use\setlength\parskip{\baselineskip]or better a class or package that is set up for such a design) – David Carlisle Aug 29 '16 at 16:20