For a general solution of lining up words the glossing macros provided by linguistics packages are ideal since they will also automatically wrap lines and keep the words lined up. The ExPex package is very good, although quite complex to use. But the built-in glossing macros of the gb4e or linguex packages will work very well for most things.
See Numbered examples in linguistics that I can refer back to for an overview of these packages.
Here's a simple example using just the cgloss4e package (documentation in the gb4e package.) The way the command works is that it takes two lines, each terminated by \\ and uses spaces to line up each word in each line. Grouping is respected, so in your example, I've grouped the first three words with braces, so that the first line effectively contains 2 "words". Then since the next line has only one word, we add an empty pair of braces as the placeholder for the first word. This correctly lines up "word" with "sentence".
I've added a longer example to show how the lines wrap automatically while retaining the correct word-to-word alignment.
\documentclass{article}
\usepackage{cgloss4e} % documentation is part of gb4e
\begin{document}
\noindent
\gll {This is a} sentence.\\
{} {word}\\
\gll This package also allows very long sentences to wrap properly and can align each word.\\
Word each align can and properly wrap to sentences long very allows also package this.\\
\end{document}

expexor the glossing macros that are part ofgb4e, especially if you need the aligned words to be able two wrap across lines. – Alan Munn Aug 08 '15 at 16:27