UNIX for Dummies Questions & Answers

1) How can I undo or redo my last action in vi editor.
2) How can I copy only a word or a portion of line (not the whole line) in vi, like
   we can select text and press ctrl+c in notepad to copy any text.
3) Suppose, I have 10 lines in a shell script, I want to comment out those lines
   i.e. I want to insert a "#" at the begining of more than one lines, how can I do that?
4) How can I join two lines to make them one, like we can press DELETE key at the
   end of first line to bring 2nd line up and make both lines a single line.