+았/었어요 vs. +았/었었어요

What is the difference between +았/었어요 (Past Tense) and +았/었었어요 (Past Perfect Tense)?

+았/었어요 (Past Tense):

To express an action or situation that simply occurred at some point in the past.

For example:

- 댄 씨는 작년에 한국에 왔어요. (Dane may still be in Korea now. We don't know what happened after he came to Korea last year. He may be in some place other than Korea now.)

- 댄 씨는 서울에서 1년 살았어요. (Dane previously lived in Seoul for one year, but we don't know where he lives now.)

+았/었었어요 (Past Perfect Tense):

To express an action or situation that was completed ("perfected") at some point in the past.

For example:

- 댄 씨는 작년에 한국에 왔었어요. (Dane came to Korea and later left. He is not in Korea now.)

- 댄 씨는 서울에서 1년 동안 살았었어요. (Dane previously lived in Seoul for one year, but now he doesn't live in Seoul.)

Source:

[1] Ahn Jean-myung, Lee Kyung-ah, Han Hoo-young, Korean Grammar in Use Beginning to Early Intermediate, page 57.