Some general principles & guidelines that work(ed) for me (I rarely instruct now) or I would often see in inexperienced instructors (when I was the Chief & teaching to instruct...

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1. New students have **very** limited capacity. They can either fly the plane....or listen to you. Not both.
Don't try to 'teach & explain' too much in the air. Explain prior to flight, demonstrate in flight, give *short* instructions to direct their actions eg one or two words at a time. YOU fly while you talk a bit about the next thing to be done.
2. New instructors tend to talk too much. You know that 'Far Side' cartoon about 'What the dog owner says: <
long list of various commands & conversation>' vs 'What the dog hears:
Blah blah blah, Rover. Blah blah blah blah blah, Rover, blahblah? That's also a good analogy to students.
Remember they're operating near/at/over their maximum capacity. There isn't much left over to attend to your epithets.
Too much talking also can make the student feel pressured, distracted & annoyed. They don't always need a noise in their ear while they practice a manouvre.
3. As someone mentioned: Always have the controls within reach near/on the ground. Not applying pressure though. I keep my feet *just* touching the pedals so I can feel whether the student is using them correctly or at all, and my hands in my lap.
In general, there are common mistakes that most students make in particular lesson in the syllabus HOWEVER every student will make a mistake that is wholly unique...
4. Try watching your student. If they're having a problem you may find the solution in where's s/he is or is not looking.
5. Following on from earlier, unload the student. Part of your job is to relieve the student of extraneous workload so they can focus on the task at hand ie devote all their necessarily limited resources to it and not dilute them with other items.
As their skill & capacity improves through the lesson - and through their course - then hand more & more back to them.
6. Be creative. Try to isolate what particular aspect of an overall problem is causing them difficulty, eliminate other tasks while those one or two items are practiced eg x/wind landings:
provided their normal landings are OK - ie landings aren't an underlying problem - then what could be stopping them from using a x/wind technique effectively? It's only an overlay on top of the more fundamental landing skill. Not enough skill at slipping? Go to the training area & practice a few then come back. Also do them in the circuit pattern. Not enough time (for the student) to establish an appropriate amount of x/wind correction in the flare? Start it earlier. Not getting the drift controlled? Fly level above the runway & have them follow through while you maintain the centreline --> slip upwind to the RWY edge --> reduce the correction to move back to the C/L --> move downwind to the other edge --> back to the C/L. Then get them to do it. If necessary you control power while they concentrate on the slipping task.
7. Think 'chaining' ie how to break up a manoeuvre into smaller chunks for initial learning, then join the chunks together to make the whole manoeuvre.
8. Don't fly if you don't feel you can handle the conditions OR teach the student effectively in those conditions. On the other hand you need to 'stretch' your abilities a tiny bit so you can improve. Just don't stretch them too much at any one time.
Whilst students can get disappointed if they don't fly if the weather isn't appropriate for that day's lesson, they'll be much happier in the long run after they've learnt you're not wasting their money for them.
They'll also carry the lesson that there are times when choosing to fly is NOT the correct decision.
9. Always, but always, demonstrate your own good standard of airmanship. DON'T talk about the rules & safety then ignore them. Monkey see, monkey do...
10. Enjoy your student's successes!!!! It's truly a wonderful feeling to see your student's happiness after their first solo, flight test etc. Especially knowing that your own skills were part of that success.