Both and and but, and to a lesser extent or and so, are common as transitional words at the beginnings of sentences in all types of speech and writing: General Jackson thought the attack would come after darkness. And he was right. Any objection to this practice probably stems from the overuse of such sentences by inexperienced writers. When one of these words begins a sentence or an independent clause within a sentence, it is not followed by a comma unless the comma is one of a pair setting off a parenthetical element that follows: John is popular, and he seems to be well adjusted. But, appearances to the contrary, he is often depressed. See also and/or, et cetera, try.