Why does LaTeX throw an error without \begin{document}?

Question

Consider the following "Hello, World!" document.

Hello, world!
\end

If you try to compile this using latex.exe, you get the error**

! LaTeX Error: Missing \begin{document}.

Okay, so on one hand, it should be obvious that one would get this error---this is a bit like trying to compile a C program without a main function. On the other hand, if I am thinking of LaTeX as just TeX with a bunch of macros loaded, one would think that any document that would compile fine using tex.exe should work just as well without latex.exe, at least so long as I'm not using any macros specific to Plain TeX.***

This suggests that my understanding of LaTeX as being effectively equivalent to TeX but with an \input{latex} command at the beginning of the document is incorrect, which thus raises a couple of related questions.

1. My understanding was that loading a TeX format is essentially equivalent to \inputing a file at the beginning of the document, except perhaps that the file has been precompiled. Is this understanding inaccurate?

2. Is a call to latex.exe actually equivalent to a call to tex.exe with the LaTeX format loaded?

3. If in fact running latex.exe is equivalent to just running TeX with the LaTeX format, then how does this format tell TeX to throw an error if it doesn't see a \begin{document}?

** Incidentally, you also get an error related to the fact that \end means something different in LaTeX. I also find this confusing because I didn't think you could redefine TeX primitives, but perhaps this is better saved for a separate question.

*** Actually, I'm not even sure if there are macros specific to Plain TeX that LaTeX doesn't also define.

Answer 1

1. My understanding was that loading a TeX format is essentially equivalent to \inputing a file at the beginning of the document, except perhaps that the file has been precompiled. Is this understanding inaccurate?

More or less true, although you need to interpret that with some care.

2. Is a call to latex.exe actually equivalent to a call to tex.exe with the LaTeX format loaded?

yes but crucially without the plain format being loaded (what was originally called virtex) and ignoring the point that hyphenation patterns can only be loaded in initex, not in a normal run.

3. If in fact running latex.exe is equivalent to just running TeX with the LaTeX format, then how does this format tell TeX to throw an error if it doesn't see a \begin{document}?

latex.ltx sets

 \everypar{\@nodocument} %% To 

so every paragraph generates an error, \begin{document} clears this and allows paragraphs to be set.

** Incidentally, you also get an error related to the fact that \end means something different in LaTeX. I also find this confusing because I didn't think you could redefine TeX primitives, but perhaps this is better saved for a separate question.

yes of course you can redefine any primitive

\def\hbox{oops}

will redefine \hbox (not a good idea but ...)

*** Actually, I'm not even sure if there are macros specific to Plain TeX that LaTeX doesn't also define.

yes there are macros defined in plain not defined in latex (\folio for example) and many many commands that are defined with the same name in both formats have different definitions (\loop for example).