I am a beginner in Houdini. I always need to turn “highlight” off when I am editing the polyon 's vertices. Is there a key for “select clear” in the view port for modeling?

Why? Perhaps you could try using Wireframe Over Shaded mode in the viewer. As for your second question, when you're prompted to select something, try using CTRL+right mouse to see what the hotkey (if any) for Select none is.