SOURCE

$(function(){
    $('span').css("color","red").css("font-size","0.7em")
    $('#btn').click(function(){
        let uname=$("input[name='uname").val()
        let pwd=$('input:password').val()
        if(uname===null || uname.length===0){
            $("input[name='uname").next('span').text("用户名不能为空")
        }
        if(pwd===null || pwd.length===0){
            $("input:password").next('span').text("密码不能为空")
        }
        if(uname==='admin'&&pwd==='123456'){
            alert('登录成功')
        }
    })

})

<script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
<form>
    <div>
        <input name="uname"/><span></span>
    </div>
     <div>
        <input name="pwd" type="password"/><span></span>
    </div>
    <input id="btn" type="button" value="登录"/>
</form>