The word "sure" is correct and usable in written English. It is typically used to express agreement or acceptance. For example, you could say "Okay, I'll do the task. Sure, I'll get it done by tomorrow.".
Are you sure?
The EU may be getting the booby prize for peace because it sure hasn't created prosperity.
I have no ideas and I'm not sure where to begin".
"I am not sure".
I am not so sure.
No one can say for sure.
"The Commission needs to make sure that consumers continue to have access to a wide variety of music in different physical and digital formats at competitive conditions".
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union