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}
