ANSWER: When “so” begins a clause:
.e.g. “I studied hard, so I am sure to pass the test.”
The same punctuation rule is used with: FOR – AND – NOR – BUT – OR – YET
…or FANBOYS to help you remember.
e.g. “It is important, but not everyone agrees with the policy.”
“There is a new rule in the workplace, yet no-one knows about it.”
If the sentence is quite short, this punctuation is often skipped.