contact form not working

I have been tampering with the contact form for many hours but it still does not work. Unfortunately, I have no knowledge of PHP.
Can you please be so good to correct the contact form?

Also very important: I do not want the textbox "username" to be present. Please remove it.

Very many thanks in advance!
PS: Hopingly, I have inserted the codes OK for.

<form action="mail.php" method="POST">
				    <div class="row">
				        <div class="col-md-6">
				            <div class="form-group">

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