Do you say stop as sdop? Or could it be that stop is just sounded out like sdop while you never actually intend to pounce it that way? We were taught, as least where I live, to say st as sd when it is followed by a vowel. There is a long list of such words, like sdeady, sdar, sdink, sdar, sdupid, websder... just to give you an example. But I always wonder how much truth there is in it. Could anyone please clarify this for me?