I'm sure it depends crucially on the student. The other thing to remember is that teachers speak to roomfuls of students at a time, not one student at a time.

I'm sure it depends crucially on the student. The other thing to remember is that teachers speak to roomfuls of students at a time, not one student at a time. If you've got one student, then sure, you need never explicitly say that a particular concept is tricky. You can determine trickiness by trial-and-error: If the student gets confused at any point, explain again, but otherwise just keep going. But with a whole roomful of people it's not as if you're going to fool them by avoiding the word "tricky": They're going to know when you cover something that is tricky, because half the audience will be confused, and the other half is then going to have to observe you explaining the tricky thing over and over again to the first half.