I think he'll be back on Monday, but I can't say for sure.

我想他星期一会回来，不过我不敢肯定。

《牛津词典》

You can't say for sure that it won't rain tomorrow.

明天难保不下雨。

《新英汉大辞典》

Researchers can't say for sure why some people tend towards greater ambivalence.

研究人员无法确定为什么某些人的纠结情绪会浓于他人。

youdao

"When you can't say for sure whether all the women in the advance provision group actually used the medication, then you're kind of looking at population level effect of a strategy, not the individual effect of a medication."

VOA: standard.2010.03.22

" Cochran confidently replied, "I can't say for sure, but I think he's looking forward to that.

CNN: O.J. Simpson Profile: The Adult Years

We can't say for sure how much the Bank Holiday or the Olympics have distorted these figures.

BBC: A special recovery

However, Brown can't say for sure whether the fall version of the AudioTron will support streaming audio content.

CNN: Move MP3s to your home stereo