"So" opens an independent clause that reports a result or a consequence.
- He raised his voice so that the rescuers would hear him.
This describes his purpose in raising his voice -- in order to be heard.
- He raised his voice, so the rescuers heard him.
This describes the result of his raising his voice -- therefore he was heard.
- LaToya studied hard so that she would get an A in geometry.
This describes her purpose in studying hard -- in order to get an A.
- LaToya studied hard, so she got an A in geometry.
This describes the consequence of studying hard -- therefore she got an A.
Student or Learner