Pardon me, Almufadado, but post #6 contains several things said in ways I believe no native English speaker would say them. Nonetheless, a few corrections, with your permission, can make it quite useful:
What is a line on a piece of paper?
A line on a piece of paper is a continuous mark drawn upon the paper's surface.
What is a line of text?
A line of text consists of words and symbols written upon a visible or imaginary line on a piece of paper or other medium.
Each word or symbol that is part of a line of text is said to be in the line of text, as any part is within a whole.
Note that the text itself is written "on the paper", i.e. upon the surface of the sheet of paper.
- Upon can be abbreviated to on, and within can be abbreviated to in.
- If A is in B, then everything on or in A is also in B, and if A is on B, then everything on or in A is also on B.
- The word line can refer either to Euclid's "breadthless length" (visible or imaginary) or to a line of text.
- When an author uses words and/or symbols to express an idea, we say the author expresses that idea wherever the words/symbols are: in a line of text on a baseline on a page in a book, etc.
From all this, we can infer that the poet "says so and so" either "
in line 3" or "
on line 3", depending on which type of "line" is meant. I would also say that if the idea is expressed not by a part of a line of text but by the whole of a line of text or several lines of text, the poet expresses the idea
with the line(s) in question, not just
in them, and the line(s) of text being
on a baseline or
on baselines, the poet says so and so
on the baseline(s).
I hope this is helpful.