<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>带表单的导航条</title>
<link rel="stylesheet" href="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css">
</head>
<body>
<!--代码-->
<div class="navbar navbar-default">
<div class="navbar-header">
<a href="" class="navbar-brand">慕课网</a>
</div>
<ul class="nav navbar-nav">
<li><a href="">网站首页</li>
<li><a href="">系列教程</li>
<li class="active"><a href="">名师介绍</li>
<li><a href="">成功案例</li>
<li class="dropdown"><a href="" data-toggle="dropdown" class="dropdown-toggle">关于我们<span class="caret"></span></a>
<ul class="dropdown-menu">
<li><a href="">企业文化</a></li>
<li><a href="">组织机构</a></li>
<li class="disabled"><a href="">联系我们</a></li>
</ul>
</li>
</ul>
<form action="" class="navbar-form navbar-left">
<div class="form-group">
<input type="text" class="form-control" placeholder="请输入关键词" />
<button class="btn btn-default" type="submit">搜索</button>
</div>
</form>
</div>
<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script>
<script src="//maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
</body>
</html>
你的<li>有些<a>标签缺少闭合标签</a>，加上就好了