Adding "date modified" to twenty ten theme

edit functions.php of the theme;

close to the end, find:

function twentyten_posted_on() {
.....
.....

change the function to something like:

function twentyten_posted_on() {
	printf( __( '<span class="%1$s">Posted on</span> %2$s <span class="meta-sep">by</span> %3$s', 'twentyten' ),
		'meta-prep meta-prep-author',
		sprintf( '<a href="%1$s" title="%2$s" rel="bookmark"><span class="entry-date">%3$s</span></a>',
			get_permalink(),
			esc_attr( get_the_time() ),
			get_the_date()
		),
		sprintf( '<span class="author vcard"><a class="url fn n" href="%1$s" title="%2$s">%3$s</a></span>, date modified: %4$s',
			get_author_posts_url( get_the_author_meta( 'ID' ) ),
			sprintf( esc_attr__( 'View all posts by %s', 'twentyten' ), get_the_author() ),
			get_the_author(),
			esc_attr( get_the_modified_date() )
		)
	);
}

i worked from the original default TwentyTen – 2010-weaver might have some modification in the code that are no more compatible with the original TwentyTen –

use your idea, seems to work great.

no reason why the changes would have triggered an error (typo?):
the actual changes were only in this part:

sprintf( '<span class="author vcard"><a class="url fn n" href="%1$s" title="%2$s">%3$s</a></span>, date modified: %4$s',
			get_author_posts_url( get_the_author_meta( 'ID' ) ),
			sprintf( esc_attr__( 'View all posts by %s', 'twentyten' ), get_the_author() ),
			get_the_author(),
			esc_attr( get_the_modified_date() )

added this: , date modified: %4$s' after </span>
and this: , esc_attr( get_the_modified_date() ) after get_the_author()

(nothing complicated, and worked for TwentyTen )

if you suspect that the error is caused just by 2010functions.php, you can ftp a fresh copy (from the 2010weaver theme or from a backup) to the same location, overwriting the ‘faulty’ file.
(file manager should also work)

if you haven’t made any substantial changes to your theme, deleting it in the file manager and uploading a fresh copy should work.

(i personally edit my themes local on my computer (dreamweaver) and ftp the changed files to the server; maybe a bit more complicated than editing the files from within wordpress, but dreamwweaver has some useful syntax higlighting and error checks; and this way i can always undo changes, if the site runs into an error)
i tend to forget the perils of editing code and getting errors, and making the site unaccessibel 🙁

PS:
if you have deleted the 2010weaver, the site should go back to the default theme and should be usable. and you should be able to re-install and activate 2010weaver.