Dare (not): (không) dám

Dám means "dare" and goes before the verb: Em không dám nói. I don't dare say it. Vietnamese uses it far more than English uses "dare". Wherever English says "I'm too scared to", "I can't bring myself to" or "I wouldn't risk it", Vietnamese says không dám: Em không dám ăn mắm tôm. I can't bring myself to eat shrimp paste.

It's mostly negative or in questions: Anh có dám nhảy xuống không? Would you dare jump in? Chưa dám means you haven't found the courage yet: Em chưa dám nói với ba mẹ. I haven't dared tell my parents yet.

Plain dám, without không, often sounds like a challenge or a reproach: Sao em dám nói vậy? How dare you say that?

Dạ, không dám is something else: a formal, rather old-fashioned reply to thanks or praise, meaning "not at all, it was nothing". You'll hear it from older people more than you'll need to say it.