linux build-in commands && become root on Ubuntu

        Linux shell has many build-in commands like 'ls' , 'cd', 'alias' and so on.  When you type "sudo cd directory", shell may alert error: cannot find cd . Why ?

        When we type a command in shell , shell will first try to find the command in its build-in commands list, if it did not find it , it will try to find it in external directories, Of course , this directories we can give them a well-called name environment variables. If we use sudo command, the shell does not find the command in build-in command list, it only research in external directories. So got error.  Someone said  it should be a cd external program implemented outside. I have no idea about this opinion.

         Another problem is , someone told me on Ubuntu root is banned. I also have no idea about this. But how to become root is a problem. It seems that I do not know root's password. After searching on the internet a long time, I find the right answer. use ":sudo su" to change to root identification.

         That's all.