A good Guro is someone who:
1.) Understands the system they're teaching and knows how to apply it in real time.
2.) Knows how to teach. This is not the same as knowing the system and knowing how to fight.
3.) Doesn't hold back when they teach. In other words, they are willing to teach everything they know at whatever pace the student is capable of. If they don't know something, they admit it rather than make something up. This assumes that the student is honorable and will not abuse their knowledge.
4.) Fosters a strong sense of "brotherhood" amongst the students, compassion for all people, honor, etc. You should be able to think of your teacher and fellow students as very good friends, if not family.
This is the standard by which I have always evaluated my teachers. I have had the good fortunate to have had 4 such teachers over the years, including my current teacher.
Respectfully,
Dave Fulton
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Full Contact Martial Arts Association.
"As iron sharpens iron, so one man sharpens another."