function warning(message) {
  alert( message );
  return false;
}

function validate(f) {
  if ( f.bedrijf.value.length == 0 ) return warning( "Please enter your company" );
  if ( f.naam.value.length == 0 ) return warning( "Please enter your name" );
  if ( f.email.value.length == 0 ) return warning( "Please enter your email address" );
  return true;
}