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# LaTeX 错误：缺少 \begin document

当你忘记包含 **`\begin{document}`** 在你的文档中。如果你没有写入，也可能会出现 **`\documentclass[...]{...}`** 在主文档的开头。

## 常见原因

**忘记包含 \begin{document}：**

此错误最基本的原因是你忘记写入 **`\begin{document}`** 在主文档的导言区之后。如果你犯了这个错误，就会生成如下所示的错误信息：

main.tex，第 5 行

LaTeX 错误：缺少 \begin{document}。

请参阅 LaTeX 手册或 LaTeX Companion 以获得解释。输入 H 可立即获得帮助。... 你现在遇到麻烦了。请尝试输入以继续。如果那不行，输入 X 退出。

要解决此错误，只需确保你已经包含了 **`\begin{document}`** 在主文档的导言区之后。参见 [这里](/latex/zh-cn/latex-ji-chu/01-learn-latex-in-30-minutes.md) 以了解更多关于如何在 LaTeX 中构建文档的信息。

**\documentclass\[...]{...} 放在了错误的位置：**

此错误的另一个常见原因是 **`\documentclass[...]{...}`** 被写在了错误的位置。它必须写在主文档的开头。如果你的导言区有单独的文件，你 **不能** 包含 **`\documentclass[...]{...}`** 在那个文件中。它仍然必须出现在主项目文件的顶部，否则只会编译导言区文件。

**主文档设置错误：**

如果你在左侧菜单中错误地设置了主文档，也可能会出现此错误。这种情况可能发生在 **`\documentclass[...]{...}`** 被写在了错误的位置，因此如有需要，你应先纠正这一点。要更改主文档的选择，请打开左侧菜单：

![Lefthandmenu.PNG](/files/92774bcd1bff9309617ff93edcc3ec422f24a57d)

并前往 *主文档* 下拉菜单：

![Maindocument.PNG](/files/d76eed428e22902fb37936d5b946a4ec8006a8f5)


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