In פרשת אמור there is a Pasuk that says אך בעשור לחודש תענו את נפשותיכם, “But on the tenth of the month, you shall afflict your souls.”
We learn that the word אך which we translate as “but” always has a special teaching. We use the word אך to teach the thirty nine מלאכות on Shabbat. And we learn permission to cook on holidays from the word אך.
The present אך is related to Yom Kippur. We are not to assume that we are automatically forgiven on the tenth of Tishrei. The אך in this case teaches that “only” if we do sincere Teshuva are we given atonement. But the day itself is only effective when it is accompanied by sincere repentance.