“forever.” The Hebrew word olam (#05769 עוֹלָם), often translated “forever,” could mean “forever” or it could simply mean for a long time. We now know that it means for a long time, but the Israelites did not necessarily know that at the time.
“afflict your souls.” This was an idiomatic phrase which meant to fast, to go without food. God commanded the people to fast on the Day of Atonement. This led to the Day of Atonement simply being referred to as “the Fast” (Acts 27:9).