This pattern is used to show a stipulation or condition.

- followed mainly by imperative or propositive forms (the more common -(으)면 doesn't have this limitation)
- is also used as sentence ending to show astonishment

Examples:
취직하거든, 저한테 알려 주세요./If you get a job, let me know.
아주 바쁘거든, 가 지 맙시다./If you are very busy, let's not go.
눈이 많이 왔거든요!/It snowed a lot!

see also -(으)면 and -다가는