My leg hurts, so I went to the doctor" SO connects the idea that since their leg HURT they went to the DOCTOR.
"I am happy, so I smile." SO connects HAPPY to SMILE
SO also can say how great an extent something is
"I was going SO fast in that car" so shows how fast. How fast? SO FAST.
"He is SO good looking" Shows that he is not just good looking but SO good looking.
"That guy is SO stupid" not just stupid, how stupid? SO STUPID.
When Americans aren't feeling happy, they sometimes say "eh, I'm so-so" It means that they could be feeling better or happier.
Because explains the why and gives a reason to a statement.
I am sad, BECAUSE it is raining. Why are they sad? BECAUSE it's raining.
It can also be in front of a sentence:
"Because of the rain, I can't play out." Why can't they play? BECAUSE of the rain.
:) Is it better now?