For sure means definitely, with certainty. I don't know for sure means you aren't certain, while the short reply For sure! sounds like a confident “of course, definitely” and works in conversation with friends and colleagues. In an essay, use with certainty instead: It is difficult to say with certainty whether prices will fall.
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know/say/tell + for sure; that's for sure; For sure! (reply = definitely)
- I don't know for sure, but I think the meeting has moved to Thursday.
- 'Are you coming to Anna's birthday dinner on Saturday?' 'For sure, I wouldn't miss it.'
- The roads will be really busy on Friday evening, that's for sure.
What changed
First versionI can't say surely when the new timetable will start.
RevisionI can't say for sure when the new timetable will start.
Surely looks like a natural way to say “certainly”, but it doesn't go after say or know. It expresses surprise or the hope that the other person will agree: Surely you remember him? To say that you aren't certain about something, use I can't say for sure.
The short reply Sure is an everyday “of course, go ahead” in answer to a request: Can you close the window? — Sure. For sure more often confirms that something will definitely happen or is definitely true. For example, if a friend says We should do this trip again!, you might answer For sure!
Your turn
The website says my parcel will arrive on Monday, but I don't know ___ because the tracking hasn't changed since Friday.
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for sure — The speaker isn't certain when the parcel will arrive: the tracking hasn't changed since Friday. To say you aren't certain, use I don't know for sure. Surely doesn't go after know, and a native speaker wouldn't say it here.
Write about a plan you're not sure about yet. Say what the plan is and explain why you don't know for sure whether it will happen.
I don't know for sure if our team trip to Kazan will happen in October, because the budget isn't approved yet.
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