Just use wpdb->insert(tablename, coloumn, format)
and wp will prepare that’s query
<?php
global $wpdb;
$wpdb->insert("wp_submitted_form", array(
"name" => $name,
"email" => $email,
"phone" => $phone,
"country" => $country,
"course" => $course,
"message" => $message,
"datesent" => $now ,
));
?>