How can parents smiles and frowns babies learn good behavior?

The answer is: Helping the babies understand what behaviors are expected of them
On average, infants develop the ability to recognize the emotion in facial expression by the age of 6 months.
By smiling and frowning everytime they conduct a certain behavior, they would able to recognize the type of behaviors that desired by their primary caretakers.

That is true , instead try to have a sit down talk with them