I think it's because I rarely construct dialogue in such a way that I'm unsure about this, but occasionally I like to break an otherwise continuous sentence with a tag. Trouble is I'm not clear on the punctuation when doing so, particularly on whether the mark immediately after the tag should be comma or full stop. When constructing dialogue with two separate sentences, I know it's a full stop, as in: I passed her the wrapped parcel. "There you are," I said softly. "I hope you like it." But when it's a single sentence being broken... I passed her the parcel. "I went to the shops today,' I began. "and bought you this." That's a terrible example. I know, but it's the best I could come up with. Anyway, question remains - should it be a full stop of comma after 'began' ?