There are two other examples of the word אך that have a special meaning related to Pesach. Both are found in the twelfth chapter of שמות. The first says אך אשר יאכל לכל נפש which means, "But, only that which is needed for food for the soul." This is the source for the allowance to cook on Yom Tov.

The second אך is much more interesting in that one certainly would not understand it without the Oral Law. The Pasuk says, אך ביום הראשון תשביתו שאור מבתיכם, "But on the first day, one needs to remove his Chametz from his home." The first day means Erev Pesach, the fourteenth of Nissan. Not only is this a novel ruling, but we say that the word אך is חלק, which means to divide. It is the only day of the year that half of the day we are permitted to eat Chametz and the other half we are not. ( The Rabbis added two hours before noon as a precaution.)

The above information is fascinating but it also demonstrates the need for the תורה שבעל פה to clarify matters for us.