"A sure thing" is different from "sure thing" If I answer "sure thing", I am saying 'of course, no problem'. "Can you go with us camping this weekend?" "Sure thing! I'd love to." A sure thing would refer to some event that is certain to happen. "He thought his team was a sure thing to win."

Yes. "Absolutely!" "Definitely!" "Sure thing!" are all the same. The meaning is "It is a sure thing", i.e. "It will surely happen". Also, "sure thing" is purely colloquial. 口語的。 EDIT: usually "sure thing" is an answer to a question (thanks damocha, fdmaxey)

there is a bad Chinese tranlation, then! "Sure thing" translate into "当然不" I'm right!