关于两个元素没有同行显示的问题

代码如下
<div class="container">

<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>" class="form-signin" role="form">
<h2 class="form-signin-heading">登录</h2>
<div class="input-prepend">
<span class="add-on"><i class="icon-envelope"></i></span>
<input type="email" name="mailbox" class="form-control" placeholder="Email address" value="<?php if (!empty($user_username)) echo $user_mailbox; ?>" required>
</div>
<div class="input-prepend">
<span class="add-on"><i class="icon-key"></i></span>
<input type="password" name="password" class="form-control" placeholder="Password" required>
</div>
<div class="input-prepend">
<span class="add-on"><i class="icon-coffee"></i></span>
<input type="text" name="verify" placeholder="Verification" style="width:11.5em" required><img src="captcha.php" alt="Verification pass-phrase"
class="img-thumbnail" style="margin:5px;"/>
</div>
<div class="checkbox">
<label>
<input type="checkbox" value="remember-me"> Remember me
</label>
</div>
<button class="btn btn-lg btn-primary btn-block" type="submit" name="submit">Log in</button>
</div>
图标和输入框无法在同一行显示，图标采用font-awesomw的样式