For Sure

This one's quick and simple. Folks say, "One thing is for sure." All they need is, "One thing is sure." In general, simple is better, shorter is better, and both are more efficient. Now if someone asks, "Are you going to take care of that project?", we often reply, "For sure." "Surely" would be correct as well. The following is common and good, "He doesn't know for sure whether he will join us."