Not clearly vs not exactly: nói không rõ / không nói rõ

You know that a describing word after the verb says how something is done: nói giỏi, speak well. To say it isn't done well, put không right before the describing word: Em hát không hay. I don't sing well. Tối qua em ngủ không ngon. I didn't sleep well last night. On the phone, Em nghe không rõ means "I can't hear you clearly". Add lắm to soften it: nói không giỏi lắm, not very well.

Không can also go before the verb, as usual. Then it denies the whole action, not just the quality, and the meaning changes. Chị ấy nói không rõ means she doesn't speak clearly: she's hard to understand. Chị ấy không nói rõ means she didn't say exactly: she left out the details. In the same way, Xe này chạy không nhanh says the car just isn't fast, while Anh Minh không lái xe nhanh tends to mean that Minh doesn't drive fast, as a habit or a choice.

So to describe how well someone does something, put không before the describing word. For the modest answer to a compliment, chưa (not yet) goes in the same place: Em nói chưa giỏi lắm.