underfull \hbox (badness 10000) in paragraph

Inserting \par\vspace{\baselineskip} would provide you with a single line gap between two paragraphs. In the following minimal example, a mock-up of a solution environment adds \par\nobreak\vspace{\baselineskip} after every \begin{solution}{<stuff>} (the \nobreak addition discourages page breaking after printing the environment header). If you don't want it this way, you can add the gap manually by removing \par\vspace{\baselineskip} from the definition of \begin{solution}:

\documentclass{article} \usepackage{array}% http://ctan.org/pkg/array \newenvironment{solution}[1] % solution environment {\par\textbf{Solution\ #1}:\par\nobreak\vspace{\baselineskip}}% \begin{solution} {}% \end{solution} \begin{document} \begin{solution}{1.2} % Line 107 \newlength{\mylen}\settowidth{\mylen}{$p \to q$}% Widest element \begin{tabular}{*{5}{|>{\centering\arraybackslash\rule{0pt}{1.05em}}m{\mylen}}|} \hline $p$ & $q$ & $p \land q$ & $p \lor q$ & $p \to q$\\ \hline T & T & T & T & T \\ \hline T & F & F & T & F \\ \hline F & T & F & T & T \\ \hline F & F & F & F & T \\ \hline \end{tabular} \end{solution} \end{document} 

You might also consider using \smallskip, \medskip or \bigskip, each of which allows for some stretching/shrinking within the text.