$name="Kumkum";
$email="kumkum@gmail.com";
$phone="3456734567";
$country="India";
$course="Database";
$message="hello i want to read db";
$now = new DateTime();
$datesent=$now->format('Y-m-d H:i:s');
global $wpdb;
$sql = $wpdb->prepare(
 "INSERT INTO `wp_submitted_form`      (`name`,`email`,`phone`,`country`,`course`,`message`,`datesent`) values ("
 $name, $email, $phone, $country, $course, $message, $datesent. ')")';

$wpdb->query($sql);

$wpdb->insert('wp_submitted_form', array(
    'name' => 'Kumkum',
    'email' => 'kumkum@gmail.com',
    'phone' => '3456734567', // ... and so on
));

<?php
global $wpdb;
$wpdb->insert("wp_submitted_form", array(
   "name" => $name,
   "email" => $email,
   "phone" => $phone,
   "country" => $country,
   "course" => $course,
   "message" => $message,
   "datesent" => $now ,
));
?>

$sql = $wpdb->prepare(
    "INSERT INTO `wp_submitted_form`
       (`name`,`email`,`phone`,`country`,`course`,`message`,`datesent`)
 values ($name, $email, $phone, $country, $course, $message, $datesent)");
$wpdb->query($sql);

$sql = "INSERT INTO `wp_submitted_form`
          (`name`,`email`,`phone`,`country`,`course`,`message`,`datesent`)
   values ($name, $email, $phone, $country, $course, $message, $datesent)";

$wpdb->query($sql);

global $wpdb;
$tablename = $wpdb->prefix . "submitted_form";

$name     = "Kumkum"; //string value use: %s
$email    = "kumkum@gmail.com"; //string value use: %s
$phone    = "3456734567"; //numeric value use: %d
$country  = "India"; //string value use: %s
$course   = "Database"; //string value use: %s
$message  = "hello i want to read db"; //string value use: %s
$now      = new DateTime(); //string value use: %s
$datesent = $now->format('Y-m-d H:i:s'); //string value use: %s

$sql = $wpdb->prepare("INSERT INTO `$tablename` (`name`, `email`, `phone`, `country`, `course`, `message`, `datesent`) values (%s, %s, %d, %s, %s, %s, %s)", $name, $email, $phone, $country, $course, $message, $datesent);

$wpdb->query($sql);

$wpdb->prepare(
 "INSERT INTO `wp_submitted_form`
   (`name`,`email`,`phone`,`country`,`course`,`message`,`datesent`)
   values ('$name', '$email', '$phone', '$country',
         '$course', '$message', '$datesent')"
 );

$wpdb->insert('table_name', input_array())

global $wpdb;
$insert = $wpdb->query("INSERT INTO `front-post`(`id`, `content`) VALUES ('$id', '$content')");

$wpdb->insert( $table_name, array('column_name_1'=>'hello', 'other'=> 123), array( '%s', '%d' ) );

$wpdb->query("insert into ".$table_name." (name, email, country, country, course, message, datesent) values ('$name','$email', '$phone', '$country', '$course', '$message', )");