I've read the other topics on this but I really cannot understand why I'm getting it in my case when it has been made by the wizard rather than by hand.
I have
\documentclass[a4paper,11pt]{article}
\usepackage{graphicx}
\usepackage{framed}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsfonts}
\usepackage{mathrsfs}
\usepackage{array}
\usepackage{enumitem}
\usepackage{amsthm}
\newtheorem*{eg}{Example}
\begin{document}
\begin{itemize}
\item[\textbf{4.}]
\end{itemize}
Let $A=[0,1] \setminus \mathbb{Q}$.\\
\\
\begin{tabular}{|l||l|l|l|l|l|}
\hline
space $X$ & $A^o$ & $\overline{A}$ & $\partial A$ & Is $A$ open? & Is $A$ closed? \\
\hline
$\mathbb{R}$ & $\varnothing$ & $[0,1]$ & $[0,1]$ & No & No \\
\hline
$\mathbb{R} \setminus \mathbb{Q}$ & $A$ & $A$ & $\varnothing$ & Yes & Yes \\
\hline
\end{tabular} \\
\\
For $X = \mathbb{R}$, $\partial A = \overline{A} \cap \overline{\mathbb{R} \setminus A} = [0,1] \cap \mathbb{R} = [0,1]$.\\
For $X = \mathbb{R} \setminus \mathbb{Q}$, $\partial A = A \cap ((\mathbb{R} \setminus \mathbb{Q})_{<0} \cup (\mathbb{R} \setminus \mathbb{Q})_{>1}) = \varnothing$.
\begin{itemize}
\item[\textbf{5.}]
\end{itemize}
Proofs are in the notes.
\end{document}
Best Answer
The underfull hbox warnings are due to
as this forces two linebreaks with nothing in between so the resulting line is "under full" and infinitely bad (badness 10000 being as bad as TeX can report).
You should avoid all use of
\\outside its use to end table rows.This has no warnings: