yl I can copy the character where cursor locates. Now I press
: to enter into last line mode, I enter
%s/. Now I want to paste the character copied by
yl. How can I do that?
You can use Ctrl+R, " to insert the contents of the default register into the Ex command line. The default register is where what you just yanked will be.
:help c_CTRL-R for details.