For example, ~구나 When you would like to speak yourself in a small sound, When you realize, finally you become to know something new that you have not know exactly or something that you want to know in your potential mind, you can use ~구나 next the main verb, for example, 이것이 새구나. 이것이 산이구나, 이것이 바다구나, 이것이 컴퓨터구나, 이것이 개구리구나, 이것이 공책이구나, 이것이 볼펜이구나, 등등, On the other hands, ~네요. For example, 아기 참 귀엽게 생겼네요! 선생님 이쁘네요! 여자친구 이쁘게 생겼네요. 등등 When you use this word, certainly there is an person that can hear your word. When you want to express an feeling from something or someone, she/he can listen what you say. but ~구나 may not have an opposite person.