I'm working on a Terza rima style poem, doing the whole A B A, B C B - thing I've gotten to the middle introducing the D - the new word to rhyme and was wondering if this new word can be the end part of the prior sentence or should it be it's own sentence?
I'd love to help but unfortunately I don't know what you mean exactly. I can help you if you PM what you are working on.
Usually it will be part of a sentence that has already started. Not necessarily the end of it; the sentence might well carry on onto the next line. It would be quite unusual for a sentence to end just before the last word of a line in a classical form, because the the line ending has a sense of finality to it. Full stops, semicolons, commas and so on tend to fall at the ends of lines. In more recent times the fashion has changed somewhat. Having the major breaks in the sentences fall at the line endings -- and rhyming line endings at that -- seems a bit thump, thump, thump to a modern ear, so nowadays good poets tend to play the sentence structure against the metrical structure, and a new sentence might well start with the last word of a line.
Sort of. That's recent for a form that originated in the 14th century. But it's more that the fashion shifted gradually over the course of the last hundred years or so. Robert Frost still did things the old way, and it's hard to imagine him doing anything other. Although the shift has been happening for a bit over 100 years, it's much more recently that it has been generally accepted and not seen as just sloppiness.