submit 的返回值为false 但表单还是会自动提交，怎么回事？

<form id="login" action="pages/login_jump.php" method="post">
             你的身份：<select name="identity" id="identity">
                      <option value="student" selected="selected">学生</option>
                      <option value="teacher">教师</option>
                      <option value="admin">管理员</option>
                      </select><br  /><br  />
             用户名：<input type="text" name="user" id="username" /><br  /><br  />
             密    码：<input type="password"  name="passwd" id="passwd" /> <br  /><br  />
             <div class="sub">
             <input type="submit" id="submit" value="submit"  />
             </div>
             <div id="warn" class="sub"></div>

           </form>

//js文件

$(document).ready(function(){
    $("#login").submit(function(){check();});
    }); //match ready()
    function check(){      
      var ident=$("#identity").val();
      var uname=$("#username").val();
      var upwd=$("#passwd").val();
      if(uname==""){
          $("#warn").text("用户名不能为空！");
          $("#username").focus();
          return false;
          }
       if(upwd==""){
          $("#warn").text("密码不能为空！");
          $("#passwd").focus();
          return false;
          }    
          $.ajax({
            type:'json',
            url:"pages/login_identify.php",
            async:false,
            cache:false,
            data:'ident='+ident+'$name='+uname+'&pwd='+upwd,
            dataType:'json',
            success: function(msg){
                  $('#warn').html("姓名:" + msg.name + "密码:" + msg.pwd);
                  return false;
                  },
            error: function(XMLHttpRequest, textStatus, errorThrown){
                     $('#warn').html("无法提交验证！");
                     return false;
                  }
            });
            
    }
//login_identify.php
<?php
//require('mysql_con.php');
echo json_encode(array ('name'=>$_REQUEST['name'],'pwd'=>$_REQUEST['pwd']));
?>