这是 MWE。很抱歉没有早点放上去。
\documentclass [11pt,letterpaper,twoside,openright]{book}
\usepackage{amsmath}
\usepackage{soul}
\begin{document}
This is the example to show that whether the use of ``underset" is correct. If underlined text is $\underset{1}{\text{\ul{one or two words}}}$ it is fine; however if it is $\underset{2}{\text{\ul{very very long text, such as this, the underlined text does not break line, see this. The line from here is not published.}}}$ \\
$\underset{3}{\text{\ul{Second request is that if the underlined text shows up in two lines,}}}$
$\underset{3}{\text{\ul{the numeral below that, in this case `3' should also show up in each line.}}}$
\end{document}