1. indicates possessive
orig. written 乃 or 之
この世界のことはよく知らないから面白いな。It's interesting because I don't know much about this world.
2. nominalizes verbs and adjectives
3. substitutes for "ga" in subordinate phrases
4. indicates a confident conclusion
at sentence-end; falling tone
5. indicates emotional emphasis
at sentence-end
6. indicates question
at sentence-end; rising tone
彼には話したの?Did you tell him?