Formal:
* কিছু না (kichu na): This is the most common and neutral way to say "nothing." It literally means "not anything."
* কিছুই না (kichui na): This is a slightly more emphatic version of "kichu na," also meaning "not anything."
Informal:
* কিছু না (kichu na): This is also used informally, and is the most common way to say "nothing" in everyday conversations.
* না (na): This is simply "no," and can be used to mean "nothing" in certain contexts.
* কিছুই (kichui): This is a shortened form of "kichui na," and is informal.
Example:
* আমার কাছে কিছু না। (Amar kache kichu na.) - I have nothing.
* তার কাছে কিছুই না। (Tar kache kichu na.) - He has nothing.
* আমি কিছু না খাই। (Ami kichu na khai.) - I didn't eat anything.
* আমি না। (Ami na.) - I don't have anything (in response to a question).
The best way to choose the right word depends on the specific situation and the level of formality.