"2, What i understand that if project is not intended to modify in future and if it is run and debug for long time then it does not requires any software maintenance."

An excellent example of "famous last words", every program I have seen that was never to change did.

"3, However I think i was unable to understand the word 'maintainable' in " I do not give a hoot if your program works if it is maintainable". Can you please explain it with example."

If a program "works" and - as usual for such - develops a "mysterious problem" you are up the creek if it is not designed maintainable.
However, if a program is designed mainatainable and does not work, it is a small task to make it work.

Erik