Why is \prevgraf zero at the end of the paragraph (or even in the middle of the paragraph)?

Question

Both \the\prevgraf show zero. I expected it to show current line counter.

\documentclass[varwidth]{standalone}
\usepackage{expl3}
\begin{document}
  \parbox{40mm}
  { \parshape 3 0mm 40mm 5mm 35mm 10mm 30mm
    This is sentence 1. This is sentence 2.
    \the\prevgraf
    This is sentence 3. This is sentence 4.
    \par
  }
  \the\prevgraf
\end{document}

Answer 1

TeX cannot know the number of already typeset lines, when in the middle of a paragraph, for the simple reason it has not yet typeset it.

When a paragraph starts up, the value of \prevgraf is reset to 0; it is updated only when a paragraph or a “paragraph-so-far” has been split into lines (the latter case happens when a paragraph is interrupted by a displayed equation).

After a displayed equation is typeset, the value of \prevgraf is updated adding 3 to the value it was assigned when typesetting the “paragraph-so-far”.

Why does your last command show 0 as well? Because the paragraph was typeset in a group (the \parbox) and so the value of \prevgraf is forgotten as soon as the group ends.

\documentclass{article}

\begin{document}

  \parbox{40mm}
  { \parshape 3 0mm 40mm 5mm 35mm 10mm 30mm
    This is sentence 1. This is sentence 2.
    \[a=b\]
    \the\prevgraf
    This is sentence 3. This is sentence 4.
    \par
  \the\prevgraf
  }
\end{document}