这样写form-group为什么不在form里

<div class="modal-body">
    <form class="form-inline" id="url_{$key}_form" method="POST">
        <div class="form-group">
            <label for="input_{$num}" class="sr-only">url</label>
            <input type="text" id="input_{$num}" class="form-control form-control-url" name="old_url_{$num}" value="{$url}"/>
            <a  class="del_url_button btn btn-sm btn-warning" onclick="delInput(this)">-</a>
        </div>
        <div class="form-group">
            <div class="modal_btn">
            <a  class="add_url_button btn btn-info" id="url_{$key}" class="btn btn-sm btn-info">+</a>
            <input type="submit" class="btn btn-default url_form_submit" value="提交"/>
            <button type="button" class="btn btn-danger" data-dismiss="modal">关闭</button>
            </div>
        </div>
    </form>
</div>

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第二行不是form 吗..

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楼上解释清楚了，只是题主没找到form