Some people behave well in front but behind they speak evil of people. (Simple Sentence)

What an intelligent creation of God the human being is on the earth!

None but man has the capacity to think, talk, and act accordingly.

Mustn't he know how to behave well and perform good manners?

Parents are bound to teach their kids how they should behave with family members, neighbors, friends, and teachers./Parents are bound to teach their kids about their behaviour with people who are family members neighbours, friends, teachers, etc.

Modesty, humbleness, kindness, and courtesy are essential traits, and a well-behaving person has these traits.

This is a bad manner. / This is a poor manner.