help with form_input()

[eluser]sanguina[/eluser]
The documentation says that to populate form field with form_input() - use:

'value' => 'enter your login id', in the array.

Also to repopulate fields in case of error in entry - we are supposed to do:

'value' => set_value('login'),

I basically want to do both -

i;e - before the user enters anything - the form field should display - "enter your user id" but supposedly he enters wrong details - the form should then be repopulated with the entered data.

How to do this ? thanks