It probably is, in this case. It's difficult to say without knowing the full context. I didn't want to get into that discussion, though, since it would probably derail the thread instead of helping Karen with her immediate problem.
Not that I usually refer to HTML5 as a beacon for semantics, but the HTML5 spec says (or at least said last time I looked) that
<p> is appropriate for marking up a stanza in a poem. (On the other hand, it also said that
<p> could mark up 'a part of a form', probably meaning a label/input pair.)
A sentence can make up a paragraph. Many, many paragraphs consist of a single sentence. The one below this one, for instance.
In this case I'd say using
<pre> is the most appropriate, although you'd probably want to use some CSS as well to change the font, since browsers generally render preformatted text in a monospaced font like Courier New (ugh!).
Often, yes. But not always, which is why there is a
<br> tag even in the Strict DTDs. A line break can, in certain cases, have meaning or affect the meaning of something. In the case of poetry this is quite common.