“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 that, when used in the context of the Day of Atonement, meant to fast, to go without food. On the Nisan Calendar of Israel, the seventh month was Tishri and the tenth day of the seventh month was the Day of Atonement. It was on that day, and that day only, that the High Priest was commanded to go into the Holy of Holies, which he did twice that day. The first time he went in to make atonement for himself, and the second time he went in he made atonement for the people (Lev. 16). God commanded the people “afflict their souls” that is, afflict themselves on that day by not eating. This was so strictly observed in Israel that the Day of Atonement was simply referred to as “the Fast” (cp. Acts 27:9).