Common LaTeX errors, and how to fix them.

LaTeX met a command it does not know.

• The command is misspelled, for example \beginn instead of \begin.
• The package that defines it is not loaded.
• A command from one document class is used in another.

Check the spelling on the reported line, then load the package that provides the command with \usepackage.

A math symbol appears outside math mode.

• An underscore or caret is used in normal text, as in file_name.
• A math command such as \alpha is written without dollar signs.
• A dollar sign that opens math mode is never closed.

Wrap the math in $...$, or escape the character in text, for example file\_name.

LaTeX cannot find a file the document asks for.

• A package is not installed in your TeX distribution.
• An image path or file name is wrong, or the capitalisation differs.
• The file is in another folder than the one LaTeX looks in.

For a .sty file, install the package with your TeX distribution. For an image, check the name and the path relative to the main file.

Text appears before the document starts.

• Words or stray characters are typed in the preamble.
• \begin{document} is missing or misspelled.
• The file being compiled is a chapter, not the main file.

Remove anything that is not a command from the preamble and make sure you compile the main file.

A table row has more columns than the table declares.

• There are more & separators in a row than columns in the tabular specification.
• A row is missing its closing \\, so two rows are read as one.

Count the columns in \begin{tabular}{...} and make every row match, with \\ at the end of each.

A brace or environment was opened and never closed.

• A closing } is missing.
• A blank line sits inside an argument that cannot contain one.
• A \begin has no matching \end.

Look just before the reported line for the unclosed brace or environment and close it.

A \cite key has no matching bibliography entry.

• The key is misspelled or is missing from the .bib file.
• The bibliography step has not run yet.
• The .bib file is not referenced by the document.

Check the key against your .bib file and build again so the bibliography is processed.

A line is wider than the text block.

• A long word, URL or formula cannot be broken.
• A wide table or image exceeds the text width.

This is a warning, not an error. Rephrase the sentence, allow a break, or scale the table or image to \textwidth.