\input vs \include

\input{file} inserts the contents of file.tex inline at the point of the command — like a literal copy-paste. \include{file} starts the included content on a new page and works with \includeonly to compile selected files only. Use \input for small fragments (preamble pieces, frequently-included snippets), \include for chapter-sized chunks.

For chapters in a book or thesis, where: (1) each chapter is large; (2) you want to compile only one chapter while iterating on it; (3) chapters should always start on a new page anyway. The corresponding \includeonly{ch1,ch3} compiles only chapters 1 and 3, dramatically speeding up iteration on long documents.

For small fragments: a custom commands file, a long preamble split into multiple files, individual figure-heavy sections, or anything that shouldn't force a page break. Also for any file inside a math environment, table, or other non-page-break-friendly context (\include forces page breaks).

\includeonly{file1,file3} in your preamble tells LaTeX to skip all \include commands except those listed. The page numbering and cross-references still work correctly because LaTeX uses .aux files from previous full compiles. After all chapters are stable, run a full compile (no \includeonly) for the final document.

No — both \input and \include automatically append .tex if needed. \input{chapter1} and \input{chapter1.tex} are equivalent. Convention is to omit the extension.

\input can be nested freely (\input inside an \input'd file). \include cannot be nested — \include inside another \include'd file fails. If you need nested structure, use \input throughout, or \include for the top level and \input for nested.

main.tex has the preamble, \begin{document}, \maketitle, \tableofcontents, then \include{frontmatter/abstract}, \include{chapters/01-introduction}, ..., \include{chapters/06-conclusion}, \bibliography{...}, \end{document}. Each chapter is its own .tex file in chapters/.

By design — \include is meant for chapter-level includes, where chapters always start on a new page. If you need an include without a page break, use \input instead. Or wrap your \include with content that doesn't have a page-break expectation.

Yes. For BibTeX/biblatex: \input{...} inside \if blocks works. Or use the standalone package's mode for switchable content. Most authors use \includeonly for selective compilation rather than runtime conditionals.