The text about the interpreting the TeX error messages is not strictly copied from the original article, but have been corrected by truly yours for consistency, and amended for clearness.

The first line of a TeX error message begins with an exclamation point followed by the text of the message. The lines that follow it show the context in which the error occurred. In the previous example, the error message indicates an Undefined control sequence. This means that TeX encountered a control sequence which was not previously defined.

The final line, which says

l.15 \section{First Section}

occurs right above the question mark prompt. It identifies the line in your document that TeX was processing when it encountered the error. The error occurred when TeX was at line 15 of the file, and that line began with

Between the first and last lines, TeX prints a detailed description of how the error occurred. This is necessary because the error may have occurred inside the replacement text of a macro that you used. When TeX encounters a control sequence, like \section, it has to look up the definition to figure out how to typeset your document. The definition of a control sequence may contain other control sequences which also have to be interpreted. It is possible for TeX to be several levels deep, as it was in this case, when an error occurs. To give the person who wrote the definition of the control sequence an opportunity to figure out what went wrong, TeX prints out a trace back of what happened before the error. As you gain experience with a particular macro package, you'll find the intervening lines more meaningful.

Correcting an error depends entirely on the nature of the error. In this case, all you need to do is correct the typo.

As stated above, there are six general classes of errors you might encounter when you run TeX. The following sections briefly describe each class.

You don't have to remember or write down the error messages that TeX produces. When TeX processes a document, it produces a transcript of everything that occurs; you can refer to this transcript later if you need to recall what errors occurred when you processed your document. TeX stores this transcript in a file which has the same name as the document and the extension .log. For example, if you process main.tex, TeX produces a transcript in main.log.

	Log files go in the current directory. TeX always places the log file in the current directory, even if you specify a path when you format your document. For example, if you process lectures/main.tex, TeX produces a transcript in main.log, not in lectures/main.log as you might expect.

Missing documents are discussed in the section called "the section called "User Files"," earlier in this chapter.

If you request a format file that cannot be found, TeX issues the following warning message:

Sorry, I can't find that format; will try the default

TeX then attempts to typeset your document with Plain TeX.[36] To correct this problem, run TeX with the correct format file name. If the format file isn't available, you will have to build it. Chapter Chapter 4, Chapter 4, describes how to build format files for several common macro packages.

Misspelling a control sequence name is one of the most common errors. For identify. In many cases, you can simply proceed after encountering this error. Although TeX may not typeset your document correctly, you can continue to look for other errors. The section called "the section called "Interpreting TeX Error Messages"," later in this chapter, describes how to continue after an error.

In some cases, TeX may become badly confused by a misspelled control sequence name, in which case you should give up and fix the spelling error before trying to process your document further. This may happen if you of the control sequences which follow.

Sometimes TeX will complain that a control sequence is undefined when you know that the control sequence is spelled correctly. When this occurs, make sure that you are using the correct format file (see the section "the section called "The Command Line"" earlier in this chapter), loading the correct macro files, and using the correct style options.

Failure to close an environment is another very common error. There are several distinct errors in this category:

TeX has a lot of operators for special treatment of mathematical formulas. These operators must occur inside mathematics mode, which is usually delimited by dollar signs in your document.

If you attempt to use math operators, superscripts, subscripts, or other math-mode control sequences outside of mathematics mode, the following error will occur:

! Missing $ inserted

This is your clue that a mathematics environment has not been closed properly or that you failed to open one before using a math-mode operator.

For example, the underscore character is usually defined to be a math-mode operator which starts a subscript, in other words H${}_2$O produces "H${}_2$O" in your document.[38] If you use the underscore outside of mathematics mode, such as in regular text:

The file "test_one" contains the …

TeX will respond:

! Missing $ inserted
<inserted text>
                $
<to be read again>
                  _
The file "test_
                one" contains the ...
?

In LaTeX, the easiest way around this problem is to enclose the offending text in a "verbatim" macro, like this:

The file "test_one" contains the ...

Note that the argument to the macrois delimited by any two identical characters (in this case, two "+" signs).

In Plain TeX or another format, the problem can be circumvented in similar ways; consult the reference for the format you are using. Also consult Table Table 1.2 in Chapter Chapter 1, Chapter 1, for a list of special characters and how to type them in your documents.

The first time that you use each font, TeX loads font metric information about the font. The font metric information, stored in a TFM file, includes information about the sizes of each character as well as kerning and ligature information. These topics are discussed fully in Chapter Chapter 5, Chapter 5.

If you request a font that does not exist, for example crm10 (a misspelling of cmr10), TeX cannot find a TFM file for the font and therefore displays:

Font \myfont=crm10 not loadable: Metric (TFM) file not found

This means that TeX attempted to associate the font described by the TFM file crm10.tfm with the control sequence \myfont, but the TFM file didn't exist. You must have a TFM file for every font that you use.

A second kind of error---actually, a warning---occurs when you are using the New Font Selection Scheme (NFSS). The NFSS performs font substitution, if possible, when an unknown font is requested. The NFSS is described in Chapter Chapter 4, Chapter 4, in the "the section called "LaTeX"" section.

There are, unfortunately, lots of other errors that can occur. Some of the errors are directly related to TeX while others are warning and error messages associated with particular macro packages. There is no way to catalog every one of them or suggest what can be done in every case.

The best advice I can offer is to isolate the problem in as small a document as possible, consult the references you have available very carefully, and, if all else fails, forward your problem to one of the electronic forums that deal with TeX (the Info-TeX mailing list and the comp.text.tex newsgroup, for example).

When TeX encounters an error, it displays an error message and a summary of the error, as described in the section called "the section called "What About Errors?"" earlier in this chapter. Following the error, TeX normally stops and displays the question mark prompt.

If you type a question mark of your own at the prompt, TeX displays the actions available to you:

? ? Type <return> to proceed, S to scroll future error messages, R to run without stopping, Q to run quietly, I to insert something, E to edit your file, 1 or … or 9 to ignore the next 1 to 9 tokens of input, H for help, X to quit. ?

You can type any of the following responses at the question mark prompt: