One of the biggest and most serious faults I see when driving instructor training, is that most pdi's allow there pupils to fill in the blanks when training. And it is primarily this that will cause you to fail on your Adi part 3 exam.

What do I mean? Well, imagine the scenario, you are going along a busy road working on pst 10 anticipation, as you approach some traffic lights they change from green to red. With a normal pupil 9 times out of 10 you wouldn't need to say anything. Why? Because the pupil has used common sense and knows that if he jumps a red light the effects are 2 fold.

1) He may end up getting nicked
2) He may end up dead

But he has done that, you have not taught it him. Your part 3 test is a test of Teaching Ability.

Therefore he has filled in the blanks for you. and you fail part 3.
 

Another classic is when working on overtaking, people who want to become a driving instructor presume that because there a double white lines and because you cannot see around the corner, that the pupil will not try to overtake.

And when I do exactly that, they are taken by surprise and don't say a word until I pull back in (obviously without actually proceeding to overtake).

   This leads to " that was a stupid thing to try" "nobody would be stupid to overtake on a bend like that"

That is until I show them several newspaper cuttings where the pictures of the wreckage of the car is so bad, you can't even tell what sort of car it was! Very often in those pictures people have lost there lives,

and the reason they lost their life was that somebody WAS stupid enough to try and overtake in a place like that.

Perhaps nobody told them the risks of overtaking on a blind bend?
Perhaps nobody told them the risks of overtaking on solid white lines?
Perhaps their driving instructor never said they must ask those 3 questions
Do I need to overtake? Is it safe? Is it legal?

The reason your pupils don't do it is they are filling in the blanks in your training and using common sense, sadly they are not doing it because you TOLD them not to! And that's why you'll fail your part 3 exam.