问wordpress实现自定义如何去除li标签以及调用function start_lvl 层及乱了？

<ul class="navbar-nav ml-auto text-center">

<li class="nav-item active">

<a class="nav-link" href="index.html">Home</a>

</li>

<li class="nav-item @@about">

<a class="nav-link" href="about.html">About</a>

</li>

<li class="nav-item @@courses">

<a class="nav-link" href="courses.html">COURSES</a>

</li>

<li class="nav-item @@events">

<a class="nav-link" href="events.html">EVENTS</a>

</li>

<li class="nav-item @@blog">

<a class="nav-link" href="blog.html">BLOG</a>

</li>

<li class="nav-item dropdown view">

<a

class="nav-link dropdown-toggle"

href="#"

id="navbarDropdown"

role="button"

data-toggle="dropdown"

aria-haspopup="true"

aria-expanded="false">

            Pages

</a>

<div class="dropdown-menu" aria-labelledby="navbarDropdown">

<a class="dropdown-item" href="teacher.html">Teacher</a>

</div>

</li>

<li class="nav-item @@contact">

<a class="nav-link" href="contact.html">CONTACT</a>

</li>

</ul>

问题：如何用wp_nav_menu利用walker自定义实现这段代码？

一个是ul替换成div的时候，层及混乱；一个是二级目录里li标签删除，只留a标签。