Arghhhhhhhh. I want to beat those types with a stick. Instructors must give something positive to each student or the student will not want to come back.
Sometimes the only positive thing you can find to say is that they actually found the airplane on the ramp and brought the keys with them, but SOMETHING. Sheesh.
That's the only way to give effective instruction. Never critique the person, critique the performance against a standard. Everyone knows they're not Mozart yet, and they know Mozart needed practice too.
Obviously later on, students themselves will say things like "that sucked" without prompting, without an explanation but the instructor had to give the explanation in clear concise terms before that for the student to know what "sucked" means.
Slang in instruction was a pet peeve of my DPE for the CFI ride. "Firewall it"... (What does the firewall have to do with this?)... "Add power..." (How? How much?)...
Try this one sometime. Try to tell someone who's never touched a control in their life how to fly "straight and level" in the least number of words possible including how to accomplish it, how to measure it, and how to maintain it.
Remember, they're 90% saturated from the time the propeller started turning until it stops at the end of the flight. They have only 10% of their brain listening to you.
It's a very interesting thought process. I'm looking forward to working with real students... who will still misunderstand me because I wasn't clear and then try to kill me.