I'm not sure / certain / I don't know (for sure) / I cannot say with any certainty whether he will come.
He is still uncertain / undecided as to what action to take. / He is still wondering what to do.
There is still considerable doubt about / as to the feasibility of the plan / surrounding the future of the project.
It is a matter for debate / debatable / It is doubtful whether we will gain anything by this.
I have my doubts (about his competence).
One can hardly expect him to agree to such terms, but you never know.
I'm (quite / absolutely) certain / sure / positive / convinced that she did it.
We are (quietly) confident that we will win.
There can be no doubt / question / It is beyond doubt / question / dispute / It is indisputable / undeniable that she is the best boss we have had.
Nobody can deny that he has great experience in this field.
There is bound to be a period of unrest.