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# LaTeX 錯誤：缺少 \begin document

當你忘記包含時，就會出現此錯誤 **`\begin{document}`** 在你的文件中。若你未撰寫，也可能會出現 **`\documentclass[...]{...}`** 於你的主要文件開頭。

## 常見原因

**忘記包含 \begin{document}：**

此錯誤最基本的原因是你忘了撰寫 **`\begin{document}`** 在主要文件的前言之後。如果你犯了這個錯誤，將會產生如下所示的錯誤訊息：

main.tex，第 5 行

LaTeX 錯誤：缺少 \begin{document}。

請參閱 LaTeX 手冊或《LaTeX Companion》以獲得說明。輸入 H 可立即取得協助。... 你現在遇到麻煩了。請嘗試輸入以繼續。若無法運作，請輸入 X 離開。

要解決此錯誤，只需確認你已包含 **`\begin{document}`** 在主要文件的前言之後。請參閱 [這裡](/latex/zh-tw/latex-ji-chu/01-learn-latex-in-30-minutes.md) 以進一步了解如何在 LaTeX 中組織文件。

**\documentclass\[...]{...} 放在錯誤的位置：**

此錯誤的另一個常見原因是如果 **`\documentclass[...]{...}`** 被寫在錯誤的位置。這必須寫在主要文件的開頭。如果你的前言有單獨的檔案，你 **不能** 包含 **`\documentclass[...]{...}`** 在那個檔案中包含它。它仍然必須出現在主要專案檔案的頂端，否則只會編譯前言檔。

**主要文件設定不正確：**

如果你在左側選單中將主要文件設定錯誤，也可能會出現此錯誤。這種情況可能發生於 **`\documentclass[...]{...}`** 被寫在錯誤的位置，因此如有需要，你應先更正這一點。若要變更主要文件的選擇，請開啟左側選單：

![Lefthandmenu.PNG](/files/5a5a818ebbfe23e845db47fcaa03d2e4f5a3e7df)

然後前往 *主要文件* 下拉式選單：

![Maindocument.PNG](/files/353c7dc83110650f20853205a530151f01e139fd)


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