Problem on the_excerpt(); function

Assuming that you want the full post content to appear only on single blog posts, and that you want the post excerpt to appear everywhere else, try something like this inside your Loop in index.php:

<?php
if ( is_singular() ) {
    the_content( 'Read More' );
} else {
    the_excerpt();
}
?>

Additional Codex reading:

• the_content()
• the_excerpt()
• is_singular()
• query conditionals