nu

Nu is a discourse particle rather than a word with a meaning: it opens a turn, presses gently for an answer, or concedes a point, exactly as English well does at the start of a sentence. It never inflects and never takes an ending. Keep it apart from do, which means so or therefore and carries real logical weight, and from ho, which is an exclamation of feeling. Zamenhof's dialogue is full of it, and using it is one of the quickest ways to stop sounding like a textbook.