I'm not surephraseI don't know for certain.“I'm not sure whether the shop is open.”I have no ideaphraseI don't know the answer at all.“I have no idea where I left my keys.”I have no cluephraseI don't know at all; informal.“I have no clue how this remote works.”I'm not certainphraseI cannot say something with confidence.“I'm not certain that we have enough time.”I'm blankingphraseI temporarily can't remember something I usually know.“I know his name, but I'm blanking.”