<form role="search" method="get" id="searchform" action="<?php echo home_url( '/' ); ?>">
    <div><label class="screen-reader-text" for="s"><?php _e( 'Search for', 'yiw' ) ?>:</label>
        <input type="text" value="<?php echo get_search_query() ?>" name="s" id="s" />
        <input type="submit" id="searchsubmit" value="<?php _e( 'Search', 'yiw' ) ?>" />     
        <input type="hidden" name="post_type" value="post" />
    </div>
</form>