네(ne)
Used to simply agree or acknowledge what someone said
When to use
Simple agreement, Phone call response, Confirming someone's statement
The nuance
Used to briefly respond 'I see' or 'Okay' to someone's statement without expressing surprise or realization.
구나(gu-na, guna)
Used when realizing a new fact or expressing admiration/surprise
When to use
Realizing new fact, Expressing admiration, Reaction of surprise
The nuance
Attached at the end of a sentence to express the speaker's realization or admiration upon learning something new; conveys emotion in reaction to someone else's words.