JavaCard OS

I have a new blank java card, which is never used before. Now I have to create MF while connecting card. For all I know, this operation-creating MF can only be done one time. After this, I can not modify it any more, is it true?

MF has been already created in every blank card. You don't have to create MF.
What you need to do is to create ADFs and EFs according to your demand.
This can be done at any time.