You got my attention with the banana and the nothing weird in the same sentence ;)
Why don't you do something like this?
HTML
<div id="output">
<div class="sentence">
<span id="subject" class="word"><?= subject(); ?></span>
<span id="verb" class="word"><?= verb(); ?></span>
<span id="adjective" class="word"><?= adjective(); ?></span>
<span id="noun" class="word"><?= noun(); ?></span>
</div>
</div>
CSS
.sentence {
text-align: center;
}
or, if you prefer to have a somewhat smaller space for your sentence, you could do something like:
CSS
.sentence {
margin: 0 auto;
text-align: center;
width: 50%;
}