You could use: Sorry , I cannot find the right word = I am looking for a specific word, but I don't know it. It slipped my mind = I almost thought of the word, but I forgot it It doesn't come to mind = I can't think of the right word. Then, the last two mean different things: I cannot get my head around this = I have tried to understand this, but I just can't It's over my head = The idea is too advanced for me, I just can't understand it. What do you say if you don't understand a student question? You could say: Sorry, I didn't quite catch that. Could you repeat it? Sorry, I don't understand what you mean. Could you repeat that? Sorry, I don't get what you mean. Could you say that again? There are lots of variations of these. Good luck! :)