Spaces (glues and kerns) disappear after line breaks by rule; by the same rule, line breaks can happen only at the "left edge" of a space. Moreover, if one says \hspace{1cm}\hspace{1cm}, the first \hspace is a feasible break point, but not the second one.
This is how TeX attains alignment to the left margin for normal and ragged right paragraphs (ragged left uses a different feature).
More precisely, a glue (\hspace) can be chosen as break point only if it is preceded by a non discardable item (most commonly a character or a rule). Among discardable items are penalties.
In some cases one wants to have spaces that don't disappear at line breaks: for instance blanks that should filled by hand in an exercise. Thus LaTeX provides the \hspace* variant.
When \hspace*{1cm} is called, LaTeX uses the internal command \@hspacer, which does
\vrule width 0pt \nobreak \hskip 1cm \hskip 0pt plus 0pt minus 0pt
\vrule width 0pt places an invisible object that takes up no space;
\nobreak sets a point where line breaking is impossible, by issuing a very large penalty;
\hskip 1cm cannot be used as a break point, because it is preceded by a discardable item (the penalty);
\hskip 0pt plus 0pt minus 0pt is inserted to avoid possible problems due to the peculiar syntax of the primitive \hskip; it won't be a feasible break point, because it's preceded by a discardable item;
Thus \hspace*{1cm} forms, so to speak, an indivisible unit that will never be taken as a break point, nor will disappear at line breaks.
A consequence of this rules is that a space after \hskip*{1cm} cannot be used as break point.
Suppose we want to define a \blank macro for leaving space to be later filled in by hand:
\documentclass{article}
\newcommand{\blank}[1]{\hspace*{#1}}
\begin{document}
\parbox{4cm}{
Some text to see what's going on: \blank{2cm} and \blank{3cm} with text after them
}
\end{document}
The result will be
(where the rules represent the \hspace*{...} commands; they won't be printed by this command, I used them just to show the effect). You can see that the space after \blank{...} is not used as break point.
With a different definition, the blanks can appear at the end of the line:
\documentclass{article}
\newcommand{\blank}[1]{\hspace*{#1}\linebreak[0]}
\begin{document}
\parbox{4cm}{
Some text to see what's going on: \blank{2cm} and \blank{3cm} with text after them
}
\end{document}
will result in
Of course the typesetting here is suboptimal, but the point is to show how the thing works.
The \linebreak[0] command just inserts a feasible break point, without stating any particular preference for it.
For the interested readers, the "rule" effect for showing \hspace* has been obtained with the following trick:
\let\TeXhskip\hskip
\def\hskip{\leaders\hrule\TeXhskip}
\hspace* is also valid at the beginning of a line:
\documentclass{article}
\usepackage[a6paper]{geometry}
\parindent=0pt
\begin{document}
\hrulefill\sloppy
This is a sentence which makes no sense.
\hspace{3cm} And so on \ldots
This is a sentence which makes no sense.
\hspace*{3cm} And so on \ldots
This is a sentence which makes no sense.
\strut\hspace{3cm} And so on \ldots
\end{document}
\strut is only a box with height and depth from ( and no width! The behaviour of \hspace* is similiar to \vspace* for the vertical shift.
Best Answer
Use the starred version of
\hspace.