如何进行联系表格验证

我对PHP一无所知，但不知何故，在谷歌叔叔和你们亲爱的Stack Overflow用户的帮助下，我已经在我的网站上用联系表格管理了他们。有一件事我仍然不知道该怎么做：

如果某些表单为空或验证失败，如何将我的php更改为不发送表单（并让用户更正某些内容）。

1. 如果至少有一个空字段要输入，则不要发送
2. 如果电子邮件地址的格式错误，请不要发送
3. 如果用户未标记"条款"复选框，则不发送

我的PHP:

    <?php
error_reporting(-1);
ini_set('display_errors', 'On');
if(isset($_POST['email'])) {
// EDIT THE 2 LINES BELOW AS REQUIRED
$email_to = "biuro@natropiegra.pl";
$email_subject = "POZIOM 1";
$email_from = "rezerwacja@natropiegra.pl";
$email_message = "Form details below.'n'n";
function clean_string($string) {
  $bad = array("content-type","bcc:","to:","cc:","href");
  return str_replace($bad,"",$string);
}
$email_message .= "Imię i nazwisko: ".clean_string($_POST["imie"])."'n";
$email_message .= "Email: ".clean_string($_POST["email"])."'n";
$email_message .= "Telefon: ".clean_string($_POST["telefon"])."'n";
$email_message .= "Płatność: ".implode(" ", $_POST['platnosc'])."'n";
$email_message .= "Miesiąc: ".clean_string($_POST["miesiac"])."'n";
$email_message .= "Dzień: ".clean_string($_POST["dzien"])."'n";

// create email headers
$headers = 'From: '.$email_from."'r'n".
'Reply-To: '.$email_from."'r'n" .
'X-Mailer: PHP/' . phpversion();
mail($email_to, $email_subject, $email_message, $headers); 
?>
<!-- include your own success html here -->
<center>
<img src="image/natropie2.png" />
  <br>
  <br>
  Thank you for contacting us.  We will be in touch.<br>
<br>
 <a href="index.html">HOME</a></center>
<?php
}
?>

我的HTML:

    <div id="form" class="java">
</div>
<div id="x" class="java">
</div>

<div id="formphp" class="java">
<div id="formtytul">
<span class="txt3">
POZIOM 1 <span class="txt1">Cena -<span class="txt3"> 79zł
</span>
</div>
<form name="htmlform" method="post" action="email.php">
<table width="561">
<tr>
 <td width="212" align="right" valign="top">
  <label for="imie"> </label>
 </td>
 <td width="337" valign="top">
    <input type="text" input size="20" name="imie" placeholder="Imię i Nazwisko">
 </td>
</tr>
<tr>
 <td valign="top" align="right">
  <label for="nazwisko"></label>
 </td>
 <td valign="top">
  <input type="text" input size="20" name="telefon" placeholder="Telefon">
 </td>
</tr>
<tr>
 <td valign="top" align="right">
  <label for="email"></label>
 </td>
 <td valign="top">
  <input type="text" input size="20" name="email" placeholder="E-mail">
 </td>
 </tr>

<div id="formmiesiac">
<select name="miesiac">
    <option selected="selected">Grudzień</option>
    <option>Styczeń</option>
        <option>Luty</option>
        <option>Marzec</option>
</select>
</div>
<div id="formdzien">
<select name="dzien"> 
    <option selected="selected">1</option>
    <option>2</option>
        <option>3</option>
        <option>4</option>
        <option>5</option>
        <option>6</option>
        <option>7</option>
        <option>8</option>
        <option>9</option>
        <option>10</option>
        <option>11</option>
        <option>12</option>
        <option>13</option>
        <option>14</option>
        <option>15</option>
        <option>16</option>
        <option>17</option>
        <option>18</option>
        <option>19</option>
        <option>20</option>
        <option>21</option>
        <option>22</option>
        <option>23</option>
        <option>24</option>
        <option>25</option>
        <option>26</option>
        <option>27</option>
        <option>28</option>
        <option>29</option>
        <option>30</option>
        <option>31</option>
</select>
</div>
<div id="formgodz">
<select name="godz"> 
    <option selected="selected">9:00</option>
    <option>10:00</option>
        <option>11:00</option>
        <option>12:00</option>
        <option>13:00</option>
        <option>14:00</option>
        <option>15:00</option>
        <option>16:00</option>
        <option>17:00</option>
        <option>18:00</option>
        <option>19:00</option>
        <option>20:00</option>
        <option>21:00</option>
</select>
</div>
<div id="formplatnosc">
<span class="txt1">Rodzaj płatności:</span><br>
<span class="txt9">
<input type="checkbox" name="platnosc[]" value="gotowka" />&nbsp;Gotówka
<input type="checkbox" name="platnosc[]" value="voucher" />&nbsp;Voucher
</span>
</div>
<div id="formregulamin">
<span class="txt9">
<input type="checkbox" name="regulamin[]" value="tak" />&nbsp;Akcpetuję
<a href="http://www.natropiegra.pl/regulamin.html" target="_blank"> regulamin</a></span>
</div>

<div id="formwyslij">
<span class="txt7">
<input type="submit" value="Rezerwuj">
</span>
</div>

</table>
</form>
</div>