Student or Learner
This is my teacher, Mr.David. ( wrong)
This is my teacher,David. ( correct)
Is it wrong if " Mr." be followed by first name? Thank you!
It is not usual to use honorifics with the first name alone. I have met it where parents want a clear distance between the child and its tutor, and do not want the full formality of using the family name, but it is not something that will be used in normal English.
In the case of your sentence you could have argued that it was your teacher's family name
It's a custom in the southern United States for someone to address an elder with whom they are friendly as "Mr. Charlie" or "Miss Sally." It's less formal than using their last name ("Mr. Smith"), but it still shows respect. Elvis Presley, a good ol' Southern boy, always addressed Sam Phillips of Sun Records as "Mr. Sam."
Ouisch, it is good for me to know this. Thank you!