Launch Emacs with emacs -Q, insert a line contains only a \t (| represents the cursor):

\t|

M-x whitespace-mode, it should looks like (_ represents visual space):

»________$

Insert 2 spaces at the beginning of the line, it should now looks like:

··»______$

But what i want is:

··»________$

I'm a little confused why Emacs doesn't display the \t and 2 spaces before separately. Is the before 2 spaces really inserted? Can anyone help me?