How to insert data using wpdb

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 ,
));
?>

Use $wpdb->insert().

$wpdb->insert('wp_submitted_form', array(
    'name' => 'Kumkum',
    'email' => '[email protected]',
    'phone' => '3456734567', // ... and so on
));

Addition from @mastrianni:

$wpdb->insert sanitizes your data for you, unlike $wpdb->query which requires you to sanitize your query with $wpdb->prepare. The difference between the two is $wpdb->query allows you to write your own SQL statement, where $wpdb->insert accepts an array and takes care of sanitizing/sql for you.