How to comment part of a line?

You could use a so-called “meta-comment”, something like this:

This is typeset. \iffalse This is not. \fi This is typeset again.

Note the position of the spaces.

LaTeX/TeX do not support mid-line comments. the % character will start a comment which will end at the start of the new line.

What you can do is split your lines, like so

Start your
% text this
way