Back when I was learning grammar and diagramming sentences, using a coordinating conjunction such as and or but to start a sentence was against all rules. Now I find out that it was probably against the rules because it was an easy way for our English teachers to make sure we didn’t have sentence fragments. The use of a conjunction to start a sentence is a good way to draw special attention to that sentence. However, it is very informal and conversational. Because of that, it won’t work in a legal brief or other “formal” writing. If you want to use a coordinating conjunction to start a sentence, make sure you are using it for emphasis and be very careful it is not just a sentence fragment. Here are some examples:
Groucho Marx wrote in his thank you note: “I’ve had a perfectly wonderful evening. But this wasn’t it.”
Tell her to return my voicemail message. Or else.
These are both very good examples of starting sentences with coordinating conjunctions for emphasis.
Tell him to come to my office. And read the report.
This is a fragment. The sentence starting with the coordinating conjunction doesn’t make sense and doesn’t need special emphasis. It is more of an afterthought.
The danger of starting sentences with coordinating conjunctions is that doing it too much quickly loses its effectiveness. I still don’t like it and change it in most documents I proofread. Whether the attorney author accepts my changes is quite another thing, but at least I’ve made my point.
So the basic rule is to use coordinating conjunctions at the beginning of a sentence sparingly for emphasis but not in a formal writing. And not when I’m proofing your work.