Differences between 'negative prefix' and ' be not' ?
Hello,
I found out there are two ways expressing negative meaning.

For example, I am not happy/ I am unhappy.
The car's not expensive/ inexpensive.
I would like to know the slight differences and how would a native speaker deal with it?

Thank you very much!
Best wishes!

Question

Differences between 'negative prefix' and ' be not' ?
Hello,
I found out there are two ways expressing negative meaning.

For example, I am not happy/ I am unhappy.
                      The car's not expensive/ inexpensive.
I would like to know the slight differences and how would a native speaker deal with it?

Thank you very much! 
Best wishes!

Answer

"Not" just means the absence of the quality, whereas the negative prefix usually indicates the opposite of the quality: unhappy means sad, but not happy does not necessarily imply sadness.