I'm afraid you can't. The construction which immediately springs to my mind is with "now that": Now that he's done that, I don't know what to do. You could also use "since", which carries a connotation of "because" as well as "from the time that", but the tenses are a bit trickier: Since he did that, I haven't known what to do. ?Since he's done that, I don't know what to do. (sounds a bit weird to me) Off the top of my head, I can't think of anything else that works as well. As I say, I would prefer "now that", which is suitable for both speech and writing.