I can't say for sure. ✬

It's hard to say for sure. ✬

Tom can't say for sure where Mary lives. ✬

Tom can't say for sure when Mary will arrive. ✬

Tom can't say for sure how many times Mary has been to Boston. ✬

I think Tom will come, but it's hard to say for sure. ✬

Tom couldn't say for sure where the kidnappers had taken him. ✬

Tom couldn't say for sure when he'd come back. ✬

I wish I could say for sure. ✬

I can't say for sure how many times I've been to Boston. ✬