Hilfe... ich kann kein Javascript!

<script type="text/javascript">


</script>
<form action="index.php?go=acp/acc_create" method="post" name="formular">
<table border="0" width="450" align="center">
<tr>
<td>Username:</td><td><input type="text" name="user" style="width: 250px" /></td>
</tr>
<tr>
<td>Password:</td><td><input type="text" name="pass" style="width: 250px" /></td>
</tr>
<tr>
<td>Ablaufzeit:</td><td>in <input type="text" name="ablauf" value="0" style="width:50px" /> Tagen</td>
</tr>
<tr>
<td>Rank:</td><td><select name="rank" size="1" style="width: 250px"><option value="u">Kunde</option><option value="f">Freund</option><option value="a">Administrator</option></select></td>
</tr>
<tr>
<td>Submit:</td><td><input type="submit" value="Account erstellen" /></td></tr>
</table>
</form>

<script type="text/javascript">
document.formular.user.disabled = false;
document.formular.pass.disabled = false;
function checkfelder()
{
  if (document.formular.rank.value == "u")
  {
    document.formular.user.disabled = true;
    document.formular.user.value = "Username wird automatisch generiert";
    document.formular.pass.disabled = true;
    document.formular.pass.value = "Password wird automatisch generiert";
    document.formular.ablauf.disabled = false;
  }
  else
  {
    if(document.formular.user.disabled == true)
    {
      document.formular.user.disabled = false;
      document.formular.ablauf.disabled = true;
      document.formular.user.value = "";
      document.formular.pass.disabled = false;
      document.formular.pass.value = "";
    }
  }
}


</script>