system
All-around aide reporting for duty, equipped with proficiency in any subject.
human
Add a comma and conjunction to a compound-complex sentence to clearly separate the clauses: The train arrived early Karen missed it.

Input:
When Karen finally got to the station, she was disappointed to learn that the train had already arrived early.
gpt
When Karen finally got to the station, she was disappointed to learn that the train had already arrived early, so she missed it.