SOURCE

$('#verification').hide();
$('#login').click(function () {
  if ($(":text").val().length == 0) {
    $('#username').focus();
    $("#errorMessage").html(
      "请您填写手机/邮箱/用户名");
  } else if ($(":password").val().length == 0) {
    $('#password').focus();
    $("#errorMessage").html(
       "请您填写密码");
    $('#verification').show();
  } else if($('#verification').val().length == 0 ) {
      $('#verification').focus();
      $('#errorMessage').html("请您填写验证码");
    }
});
$('#verification').blur(function () {
  if ($('#username').val().length != 0 && $('#password').val().length != 0) {
  if ($('#verification').val().length ==0){
    $('#errorMessage').html("请您填写验证码")
    } else if ($('#verification').val() != 'yzm'){ 
    $("#errorMessage").html(
      "您输入的验证码有误:" + $('#verification').val());
  }else {
    $("#errorMessage").html('');
  }
}
});

<p id="errorMessage"></p>
<input id='username' type="text" name="username">
<input id='password' type="password" name="password">
<input id='verification' type="text">
  <br>
<button id='login'>登录</button>

#errorMessage{
  color: red;
}