A sentence should have a beginning, middle, and end. I've never liked that theory because it doesn't help. Are struggling students turning in sentences with no beginning, middle, or end? Not really. They're turning in sentences and paragraphs with bad and/or unrelated beginnings, middles, and ends. The real difficulty with prose is flow. What connects the beginning, which is by definition there even in the worst sentence, to the middle, which is also by definition there? Does the ending make some point worth making that flows from the combination of the beginning and the middle? Or is it simply the last few words that happen to be there?
An aspiring artist asks, "How can I draw a good portrait?" He receives the response, "A good portrait has a top, a middle, and a bottom." Or, "Young composers, hearken to me: to write good music, be it a commercial jingle or a symphony, you need to make sure it has a beginning, a middle, and an end." Wow, that really clarifies things for me! The writing theory of beginnings, middles, and endings has always struck me as akin to the joke about Michaelangelo saying be began with a block of marble and then chiseled off everything that didn't look like David. True, in a way, but hardly helpful. You can't hand a manuscript back by saying, "I'm sorry, this paper has no beginning." Of course it does. It just has an abrupt, unclear, or boring beginning. Fixing that is not a matter of saying it needs a beginning, a middle, and an end. The paper also has a middle, possibly a redundant, contradictory, or irrelevant middle, but a middle nonetheless. Again, it doesn't need a middle, it needs a worthwhile middle.
Of course, I get the point being made by the whole "beginning, middle, end" theory of sentences, paragraphs, chapters, and works. But addressing disorganization this way leads to overly-formulaic speeches. Sometimes an abrupt ending drives a point home, other times several rephrasings and repetitions drive it home. Sometimes a good beginning sounds like launching into the middle and picking up a story as though it is ongoing. "Fred was hanging for dear life, clawing at the rim of the canyon, when he realized..." Other times telling people what you're about to tell them works better as a beginning. "I'd like to tell you about how important it is to realize that every day may be your last. To do so, let me first offer you the example of Fred and the day a near death experience opened his eyes to..." But to recommend a beginning, middle, and end does little more than advise the speaker, "First, starting talking. Then, keep talking. Lastly, stop talking. See? Making a speech is easy!"