Some pupils overuse the comma and use it to connect two independent sentences.
This grammatical error is called a comma splice.
Here is an example:
The movie is excellent, it shows us how the aboriginals lived, first of all the movie is based on a true story. It is told by aboriginals who escaped the camp and still live today, everything that happened there must be true, we get an impression of how the settlers were treated through the eyes of the aboriginals.
Pupils who need to work with comma splice can check out this document here and work with exercises here.
Here is the source for the picture used above..