I don’t post often but when I do it has to be with this kind of question I guess.
Yes you can.
You can be certain the day after today (Monday) is going to be Tuesday. Today will turn into tomorrow which will in turn actually be today.
You can be certain at some point you are going to die. Its a fact of life. I honestly can say with certainty at least in my lifetime that we won’t be able to cure humanity enough for immortality to become a thing. You can say but how can you be certain and I’ll say our President is Donald Trump.
Of course you can be sure of absolutely everything that doesn’t mean that what you are sure of will come about. You can be sure you’ll wake up tomorrow but if you die in your sleep who is going to tell you that you were wrong?
The question isn’t “are the things you certain of actually going to happen?” its if you can be certain of anything and people can be certain of all kinds of things and whether they happen or not doesn’t matter.
You also have people that don’t like to be certain of anything. They don’t like to believe that things will work out and that’s their choice. They might be happier when proved right but that doesn’t mean that they are right.
End of the day if people are certain of things then they are certain of them.