定規 and 紙: a drawing tool and a surface
Separate tool, surface, and result in sentences about working on paper with a ruler.
Purposes of a ruler
定規, read じょうぎ, means ruler. 定規で長さを測りました。 says that someone measured a length with a ruler.
定規を使ってまっすぐな線を引きました。 means drawing a straight line using a ruler. Here it guides drawing rather than measuring.
Paper as a surface
紙, read かみ, means paper. 紙に丸を描きました。 means drawing a circle on paper. 紙に marks the drawing surface.
紙を半分に折りました。 means folding paper in half. It does not mean cutting it or separating it into two sheets.
Three roles together
定規で紙に線を引きました。 means drawing a line on paper with a ruler. で marks the tool, に the surface, and を what was drawn.
紙を二枚重ねました。 means stacking two sheets of paper. 二枚 counts sheets rather than expressing length.
What are the tool and surface in 定規で紙に線を引きました?
Show the answer and why
The ruler is the tool, paper is the surface, and the line is what was drawn.