Not at all, never ever: không hề / chưa hề

Hề between không and the verb makes "not" absolute, "not in the least", "not at all": Tôi không hề biết chuyện đó. I had no idea about that at all. With chưa (not yet), it's "never once, up to now": Em chưa hề đi máy bay. I've never once been on a plane.

It sounds firm and serious. You'll meet it often in the news, in statements and in stories: Công ty không hề thông báo. The company gave no notice whatsoever. It also goes before adjectives, often in reviews: Giá không hề rẻ. It isn't cheap at all.

There are other ways to say "not at all". Không … đâu, which you know, reassures or turns something down: Không sao đâu. Không … chút nào puts "at all" at the end and is the everyday spoken one: Không vui chút nào. Không hề comes before the verb and is the strongest and most serious of them. For a plain "never", chưa bao giờ and không bao giờ are the everyday words.