One of the 39 forbidden activities on Shabbos is building, or boneh. As we will see this prohibition is far broader than actually building a structure. Rather, it will include scenarios that involve fixing a structure, even in a minor fashion, adding to a structure, and as we will see, is not limited to structures. That being said, we will see a distinction made between structures and free-standing items.
Two common examples of boneh would be fixing a door that came off its hinges or even a sliding door that entirely came off its track. Another example would replacing the filter from the wtaer spout. Doing either of these activities would be prohibited on Shabbos.