Validate a textbox when a certain letter is entered

I'm trying to validate a textbox when certain letters are entered, For example, if b,c,d,e,g,y, or z are entered in the textbox a message will display - "Tell me why you entered this letter".  Then it should focus in a textarea to explain why they entered one of those letters.  After they explain why they entered that letter, the form should submit.  

If they enter a letter besides b,c,d,e,g,y, or z the form should submit without focusing on the textarea for a comment.  

There has to be a better way to write the IF statement.  Instead of writing all the IF statements below, how?  

This is what I have so far.  Please show me how to do this the right way.  Thanks


<SCRIPT  language="JavaScript">
function TheForm () {
var errors = "";
if (document.submitForm.letters.value == "b"){
    errors += "\n\tLetter  - Tell me why you entered this letter. \n";      
      }
if (document.submitForm.letters.value == "c"){
    errors += "\n\tLetter  - Tell me why you entered this letter. \n";      
      }
if (document.submitForm.letters.value == "d"){
    errors += "\n\tLetter  - Tell me why you entered this letter. \n";      
      }
if (document.submitForm.letters.value == "e"){
    errors += "\n\tLetter  - Tell me why you entered this letter. \n";      
      }
if (document.submitForm.letters.value == "g"){
    errors += "\n\tLetter  - Tell me why you entered this letter. \n";      
      }

if (document.submitForm.letters.value == "y"){
    errors += "\n\tLetter  - Tell me why you entered this letter. \n";      
      }

if (document.submitForm.letters.value == "z"){
    errors += "\n\tLetter  - Tell me why you entered this letter. \n";      
      }

<!--There has to be a better way to write the IF statement.  Instead of writing all these IF statements, how?-->


if (errors != ""){
                        
errors += alert(msg + errors + "\n\n");
return false;
}
return true;
}
</script>      

<form method="post" name="submitForm" action="send.asp" onSubmit="return TheForm();">
<input name="letters1" type="text" id="letters">
<textarea name="theComments" cols="80" rows="12"></textarea>
</form>