Why cannot I use the fdisk command ??

Why cannot I use the fdisk command ?? -- Linux general technology-Linux technology and application information. For details, refer to the following section. I have learned a lot of things that Baidu cannot do thanks to the help of my friends. One of the questions is, "Why can't I use fdisk/dev/sda or fdisk/dev/hda" and the file cannot be opened ???

Another thing I want to confirm is that both books and online friends say that the hard link of a file is to add associated data to its directory. That is to say, a 1g file, even if I build 100 hard links, does it only occupy 1 GB? That is to say, a 2G hard drive can have a 1g file and its 100 hard links?

Please kindly advise