Parshat בא is the first Parsha that contains numerous laws. Most of them relate to the קרבן פסח as well as the holiday of Pesach.
The word “אך”, which is simply translated as either, “but” or “except for.” When this word appears, it has special important meaning attached to it.
The word אך appears twice in פרשת בא. The first time it is written אך ביום הראשון תשביתו שאור מבתיכם. But on the first day, all Chametz should be removed from your home. The Rabbis say “אך חלק”, that אך is coming to divide. We are referring to the unique day of Erev Pesach where we divide the day so that we are allowed to eat Chametz in the morning but not in the afternoon. This is a pretty important אך.
The second אך says אך אשר יאכל לכל נפש, only that which is needed for food for the soul is permitted on Yom Tov. This is the source for our being allowed to cook on Yom Tov and do activities necessary for the preparation of food. We are allowed to slaughter on Yom Tov as well as knead the dough and use fire for cooking.
So we have two very important Halachot connected with the word אך.