So x Then

Observe o uso dos dois na piada abaixo:

Attending a wedding for the first time, Little Susie whispered to her mother,

"Why is the bride dressed in white?"
"Because white is the color of happiness, and today is the happiest day of her life."
Little Susie thought about this for a moment, then said
"So why is the groom wearing black?"

Os dois significam então

Then carrega também um significado de naquele momento, ou logo após, como se remetesse a uma sequência.
Let me finish this first, then we'll go.

So traz o sentido de nesse caso, por essa razão, consequentemente.
I was feeling very sick so I went to see the doctor.