What exactly do you mean by private? Unless you have shared a project, (right click project, Share) then it's 100% private. Only you unless otherwise shared (all tasks within a shared project can be viewed) can see your tasks.
If referring directly to shared projects, then there is no way to make one of those tasks private, unless moved to another non shared project.
Hi Brendon, I refered to the second paragraph you posted as an answer.
I wanted to make private one task out of a template or project because this
way, when I duplicate it and explain my members what to do in the task,
I can write my personal ideas without them getting to see them, but at the
same time they can see what needs to be done. I hope I explained myself...
Please make this suggestion to the developers as it´ll be nice to have this
Thanks in advance, Joaquin
Thank you for your feedback. You're welcome to request such feature in our Votebox: https://todoist.com/Vote/
Thanks David, will do that ;-)