When you DO need a comma:
* "Hence" introduces a consequence or result:
* "The weather was bad, hence we decided to stay inside."
* "She studied hard, hence she passed the exam."
* "Hence" acts as a transitional word, indicating a logical connection:
* "He was late, hence the meeting started without him."
* "The evidence was inconclusive, hence the case was dismissed."
When you DON'T need a comma:
* "Hence" is part of a phrase that modifies a noun:
* "The hence-forth-famous scientist made a groundbreaking discovery."
* "Hence" is used as an adverb meaning "from this place or time":
* "We traveled hence to a distant land."
In summary: Use a comma after "hence" when it introduces a consequence or acts as a transitional word. Otherwise, it's usually fine to omit the comma.