This is probably the action you will do the most. To mark an entry as checked, just click the box right to an entry. This will move the entry down to the checked list. Click the now checked box again, to re-add it to your shopping list. The checked list is sorted by last checked on top, so when you accidentally mark an entry, you will find it at the top of that list. You can also re-add all entries at once with the button above the last checked list.